Carbromal is metabolized extensively in humans. The major metabolites known to date are bromoethylbutyramide, ethylbutyrylurea and inorganic bromide. After ingestion of a therapeutic dose of 1.0 g carbromal (4.2 mmoles) by four healthy volunteers highest concentrations in serum were found to be for carbromal 30 mumoles/l, for bromoethylbutyramide up to 20 mumoles/l and for ethylbutyrylurea 2--3 mumoles/l. In patients acutely poisoned by carbromal-containing sedatives serum concentrations measured were in the range of 200 mumoles/l carbromal, 350 mumoles/l bromoethylbutyramide and 50 mumoles/l ethylbutyrylurea. These patients were comatose, apneic, had isoelectric encephalographic records and decreased body temperature. The degree of central nervous depression as judged by clinical signs was found to correlate with the serum concentrations of carbromal and of bromoethylbutyramide. Pharmacological activity and acute toxicity of carbromal and its two metabolites were examined in rats and compared with the activity of phenobarbitone. For intraperitoneal injection LD-50 values were found to be for carbromal 1.8 mmoles/kg, for bromoethylbutyramide 1.5 mmoles/kg, for ethylbutyrylurea 5.0 mmoles/kg and for phenobarbitone 0.9 mmoles/kg. Carbromal and bromoethylbutyramide severely decreased body temperature. The relative narcotic activity was estimated to be for carbromal = 100; bromoethylbutyramide = 66; ethylbutyrylurea = 33; phenobarbitone = 100. The anticonvulsive activity against pentetrazol-induced generalized seizures was nearly identical for carbromal, bromoethylbutyramide and phenobarbitone. Anticonvulsant activity of ethylbutyrylurea was two to three times less than that of carbromal. Inorganic bromide was found to increase the narcotic activity of carbromal and of bromoethylbutyramide. The findings show that the clinical signs of central nervous system depression seen in patients acutely poisoned with carbromal are caused mainly by unchanged carbromal and by its metabolite bromoethylbutyramide.