Motor disturbances observed in Brattleboro rats (homozygous for the diabetes insipidus (DI) trait) following a first intracerebroventricular injection of 1.0 microgram of arginine vasopressin (AVP) were not significantly different from those of the parent Long-Evans (LE) strain. These disturbances consisted of periods of staring and immobility, locomotor difficulties and discrete myoclonic jerks, often followed by scratching behavior. Following a second central injection of 10.0 ng AVP, however, the DI strain exhibited more pronounced motor disturbances than the LE strain. This increased sensitivity of the DI strain to the behavioral actions of AVP does not appear to be due to a generalized decrease of seizure threshold, since no significant differences were observed between the two strains in their susceptibility to convulse following pentylenetetrazol. As the behavioral and motor effects of AVP appear to be receptor-mediated, these findings suggest that homozygous DI rats possess central AVP receptors, which, in the absence of endogenous vasopressin, may have increased sensitivity to a second central injection of AVP.