Certain autochthonous breeds of donkeys (Andaluza, Catalana, Mallorquina, Encartaciones and Zamorano-Leonesa) have suffered important and frequent quantitative variations throughout the time, that generally have been negative, probably due to the intense mechanization of the country-side and the introduction of other species of asses. The decrease of the census of their populations and the indiscriminate mixture with other breeds have lead them to the state of "endangered breeds" (RD 1682, 1997), and have done confuse their ethnic characteristics. The main objective of this thesis is first of all, reflect the present situation of the Spanish donkey breeds and in the other hand, make the biometrical, haematological and biochemical characterization of their endangered populations. This work has followed the rules marked by the FAO Expert Consultation for the identification of possible stocks for conservation and as basic tool for the study, maintenance and conservation of animal genetic biodiversity. For the biometrical study 317 adult animals (older than 3 years) have been sampled and a total of 26 corporal measures (cephalic, trunk and extremities) for each individual were taken. The results obtained from these measurements provided us important information to differ individuals from others, to group them in specific sets and, mainly, to deduce proportions that indicate functional aptitudes. We obtained these proportions analysing 12 corporal indices (ethnological and functional), which demonstrates the existing relationships between some elements of compactness, height, length and weight. For the haematological, biochemical and plasma proteins study, blood samples have taken from 491 animals of both genders and age (young and adult). The description and analysis of the average, variance, and age, sex and race factors provided us guaranteed reference values to be used for the racial characterization and for the clinical scope. The correlation's analyses between the 26 corporal measures, for each one of the breeds and genders, allowed identifying the existing interactions between and within the different corporal regions (trunk, extremities and head). The factorial analysis of principal components (ACP) allowed to observe the existing relationship degree between these breeds, and to determine the most important variables of the morphological definition, with the purpose to reduce the number of variables to be used in the practical and routine characterization of the animals in later studies, reducing then the complexity of them. To do it, all the information of the 26 corporal variables was subject to a multivariate analysis, in fact, subject in a canonical analysis, where the variables were transformed into canonical variables (factor I, II and III). These three factors summarized 99.71 % of the information by the original 26 variables, the factor I contributed in 95.85 %, factor II in 3.25 % and factor III in 0.61 % to the total of the explained variance. The thoracic perimeter, length diameter, withers height, sacrum height, root of tail height, back height, pelvis height, and rump height, the back-sternal diameter and head length, mainly determined factor I, being the heaviest weight variables for the racial characterization. On the other hand, using the Mahalanobis Distance, and also with ACP, we obtained the divergence level between the 5 donkey populations studied, according on quantitative characters. The results showed the existence of three groups, without clear evidence of clustering by geographic distance. For this reason, we must suppose that phylogenetic and morphologic evolution of the peninsular donkeys have been a complex process, including different patterns of differentiation for the different characters groups, probably coming from the environmental action and to unequal selective pressures. The greater differences were found between the Andaluza and Encartaciones breeds, while the minors were located between the Zamorano-Leonesa and Mallorquina. Using the ACP and also in the Mahalanobis Distance, we observed that the Zamorano-Leonesa breed is the one that presents minor morphologic distances with the others. The analysis of breed factor of 15 haematological parameters, 11 biochemical parameters and plasma proteins indicated the existence of statistically significant differences between the 5 populations analyzed. Age had the most influence on blood parameters because the differences found between young and adult animals were remarkable (11 of 15 haematological variables and 7 of 11 biochemical ones presented significant differences). In contrast, less significant differences between genders were found for sex factor so we demonstrate the existence of a low sexual dimorphism for haematological and biochemical parameters.