To investigate the physiological role of somatostatin in the GH release mechanism, the effect of passive immunization with somatostatin on basal and poststress serum GH levels was examined in rats. Sheep antiserum to somatostatin or normal sheep serum (NSS) was injected iv into adult male rats 5 days, 3 days and 1 day before applying stressful stimuli. Thirty min after the first bleeding under ether, the mean serum GH level increased from 36.3 to 76.1 ng/ml in the antiserum-treated rats and decreased from 63.7 to 16.8 ng/ml in the NSS-treated rats, but the difference for either group was not significant due to the large variation. In another experiment, electroshock was applied to conscious rats. The mean poststress GH level in the rats pre-treated 3 h previously with the antiserum was 34.4 ng/ml, a value significantly higher than the GH level in the rats pretreated with NSS (0.9 ng/ml) or in non-treated rats (0.5 ng/ml). It was concluded that the stress-induced decrease of GH secretion in rats is at least partly due to an increased release of hypothalamic somatostatin and/or immunologically related GH release-inhibiting hormone(s). Persistent variation in serum GH levels after neutralization of endogenous somatostatin by the antiserum suggests that another regulator, probably GH releasing hormone, also plays a role in controlling GH secretion.