In nineteen rats the cardiovascular responses to the electrical stimulation of the lateral mesencephalic reticular formation (MRF) were recorded before, during and after different doses of sodium pentobarbital injected intravenously. Before pentobarbital, stimulation of the MRF induced cardiovascular changes in 100% of cases. The following four patterns were observed: a) hypertension plus bradycardia (N = 11; 57.9%); b) hypertension without chronotropic effects (N = 4; 21%); c) hypertension plus tachycardia (N = 3; 15.8%), and d) hypotension without chronotropic effects (N = 1;5.3%). Administration of small doses of sodium pentobarbital induced drastic variations in the incidence of the aforementioned patterns of response as follows: a) N = 3; 15.8%; b) N = 2; 10.4%; c) N = 11; 58%; d) N = 0. Other two complementary patterns appeared: e) no changes in blood pressure plus bradycardia N = 1; 5.2%, and f) lack of response: N = 2; 10.4%. When the cumulative doses were over than 19 mg/kg no systematization was possible due to the extreme variability of the responses. This change in the characteristics of the cardiovascular effects of the stimulation of the MRF is supposed to be due to a demasking effect of barbiturates which depressing reticular neurons allows a cardioacceleratory component of the response to become apparent.