Mice were continuously administered with barbital by the sc implantation of 16 mg barbital pellet (acid form). Serum and brain barbital levels were assessed at different times after pellet implantation. After 3 days of implantation the barbital pellet was removed and the mice were observed for a 96 hr period. A 30% decrease in sleeping time was observed 72 hr after pellet removal when mice were challenged with 400 mg/kg of barbital ip. Brain and serum barbital levels upon awakening were also assessed with a dose of sodium barbital, 400 mg/kg ip, administered 72 hr after removal of either a barbital or placebo pellet. It was observed that serum and brain barbital concentrations were significantly higher in barbital treated animals than in controls. The effect of barbital pellet implantation on pentobarbital (75 mg/kg, ip) sleeping times was also assessed. Cross tolerance to pentobarbital was observed 24, 48, and 72 hr after barbital pellet implantation. The results demonstrated that barbital pellet implantation enhanced hepatic drug metabolism as evidenced by significant increases in hepatic microsomal N-demethylase and hydroxylase activities. CNS hyperexcitability, an indication of dependence, was assessed by the method of pentylenetetrazol (PTZ) induced convulsions (60 mg/kg, sc). At 48 and 72 hr after pellet removal, PTZ induced convulsions increased by 41% and 35%, respectively, over control values. Index of dependence was also substantiated by a decrease in threshold for electroshock. These studies substantiate the validity of this experimental model for the assessment of the development of tolerance to and physical dependence on barbiturates.