The pain threshold of rats previously selected according to ambulation and defecation scores was assessed. Ss were 89 Sprague-Dawley rats aged 90 days. They were submitted to a low-frightening open-field (LFOF) test and 40 rats were selected on the grounds of extreme ambulation and defecation scores. The pain threshold was then assessed in 10 daily tests during 2 consecutive days. The mean of the 20 tests was used as the measure of the threshold and the mean of the two most extreme values was used as the measure for pain tolerance. The results obtained show that the pain thresholds of high-defecatory rats are significantly lower than those of low-defecatory rats. A similar relationship is found between low- and high-ambulatory rats, but the differences do not reach statistical significance. When the results of all 4 groups are compared the low-ambulatory and high-defecatory rats (introvert-neurotic) show the lowest threshold value, and the high-ambulatory and low-defecatory rats (extravert-stable) show the highest one which is significantly different from any other group. The results obtained with respect to the tolerance threshold, in spite of the lack of statistical significance, follow the same direction as those shown by the pain threshold. © 1986.