The aim of the present study was to determine the extent to which vasopressin or the renin-angiotensin system contributed to the recovery of blood pressure following acute hypotension induced by treatment with pentolinium and captopril, or pentolinium and the vasopressin antagonist d(CH2)5DAVP, respectively, in conscious, free-moving rats treated 21 days previously with saline or streptozotocin (STZ) (60 mg/kg ip). Half the animals given STZ were subsequently treated with insulin (about 4.5 U/day). The STZ-treated animals demonstrated a resting bradycardia and systolic hypotension. The vasopressin-mediated recovery in blood pressure seen following administration of pentolinium, in the presence of captopril, and the renin-angiotensin-mediated recovery seen following administration of pentolinium, in the presence of d(CH2)5DAVP, were both found to be significantly (P less than 0.05) attenuated in the STZ-treated animals. These abnormalities were absent in the animals injected with STZ and treated daily with insulin, but receiving their last dose 24 h before measurement. At that time the animals had elevated blood glucoses. These results indicate that the abnormalities observed were not due to toxic effects of STZ or to hyperglycemia per se.