To investigate the effect of diabetes on the sensitivity of the central nervous system to the hypnotic action of a barbiturate, studies were conducted on adult female Lewis rats made diabetic by injection of either streptozotocin or alloxan. The animals then received a slow i.v. infusion of phenobarbital (PB) until the onset of a defined pharmacologic effect [loss of righting reflex (LRR)] and the PB concentrations at that time in serum (total and unbound drug), brain and cerebrospinal fluid (CSF) were determined. In the experiments on rats with streptozotocin-induced diabetes, animals not treated with insulin had significantly lower serum concentrations of total PB at onset of LRR than did animals treated with insulin and nondiabetic control rats. Otherwise, there were no significant differences in PB concentrations between untreated diabetic and control animals. Additional experiments on untreated diabetic rats showed that, as in normal rats, the PB concentrations in CSF (but not in serum and brain) at onset of LRR were independent of PB infusion rate over a 10-fold range, indicating that PB equilibrates very rapidly between CSF and receptor sites. Experiments in rats with alloxan-induced diabetes showed no significant differences between untreated diabetic, insulin-treated diabetic, alloxan-nonresponding and nondiabetic control rats with respect to PB concentrations at onset of LRR in serum (total and unbound drug), brain and CSF and in serum protein binding. These results show that the central nervous system response to the hypnotic effect of PB is not significantly affected in two different experimental models of diabetes.