The relationship between open-field ambulation and intracranial self-stimulation (ICSS) at the lateral hypothalamic area was studied in rats. A treatment with i.p. saline of d-amphetamine was also applied to the animals prior to ICSS tests. High-ambulatory rats (comparable to human extraverts) showed an equally higher susceptibility to ICSS reward than low-ambulatory rats (comparable to human introverts) with both saline and amphetamine. These findings were interpreted comsidering the ICSS as a strong stimulation which would tend to produce 'unconditioned inhibition' much earlier in low-ambulatory rats than high-ambulatory rats thus leading to a better conditioning among the high-ambulatory Ss. © 1985.