Arterial and venous concentrations of phenobarbital, phenytoin, clonazepam, and diazepam were followed for 12 hr after a rapid intravenous injection. Initial blood samples were collected at intervals of only 5 sec. The arterial curves were initially much higher and steeper than the venous. For all four drugs the arterial curves could be described as triexponential functions. The initial part of the arterial curves were used in determining the initial supply to and uptake rate in the organs.