1 Three injections of cannabis extract (500 mg/kg s.c. given over 3 or 5 days) diminished thymus gland weight but not the weights of spleen or liver in weanling female and adult male mice kept at room temperature.2 Both cannabis extract (500 mg/kg s.c.) and Delta(1)-tetrahydrocannabinol (Delta(1)-THC; 10 and 20 mg/kg i.p.) elevated corticosterone levels in mouse plasma.3 A pretreatment that consisted of three daily subcutaneous injections of 500 mg/kg of cannabis extract and that was shown to produce tolerance to the ;cataleptic' effect of Delta(1)-THC (2 mg/kg i.v.) in mice, also produced tolerance to the effect of Delta(1)-THC (10 mg/kg i.p.) on corticosterone levels in mouse plasma. However, this pretreatment did not reduce the rise in plasma corticosterone concentration produced by immobilization.4 Tolerance to the effect of Delta(1)-THC (10 mg/kg i.p.) on corticosterone levels in mouse plasma was also produced by the pretreatment of mice with a single injection of Delta(1)-THC (10 mg/kg s.c.). Three daily injections of Delta(1)-THC (10 or 30 mg/kg s.c.) also produced tolerance.5 In a thermoneutral environment (30-32 degrees C) in which cannabis extract does not produce hypothermia, the drug no longer reduced thymus gland weight. However the effect of cannabis extract and of Delta(1)-THC on corticosterone plasma levels was the same at room temperature as at 30-32 degrees C. Tolerance to the latter effect of Delta(1)-THC was also produced equally readily under the two conditions.6 It is concluded that pretreatment with cannabis extract or Delta(1)-THC can produce tolerance to the effect of Delta(1)-THC on corticosterone levels in mouse plasma and does so without impairing the effect of immobilization stress on corticosterone release. In addition, both the rise in corticosterone plasma levels produced by cannabis or Delta(1)-THC and the development of tolerance to this effect can still take place in the absence of hypothermia.