Effects of simultaneously administered salicylamide (SAM) on absorption, distribution, metabolism and excretion of amobarbital (AMB) and pentobarbital (PEB) were studied. Absorption of AMB from the stomach was depressed by simultaneously administered SAM, but absorption of PEB was not affected by SAM. Absorption of either of them from the small intestines was not affected by the administration of SAM. Movement of AMB or PEB to blood, brain, kidney and liver was markedly depressed by the administration of SAM. Intraveneous injection of AMB or PEB after intraperitoneal injection of SAM increased the blood level of AMB and PEB. The excretory rate of AMB into urine was decreased, and the quantity excreted in 24hours was increased by the administration of SAM. On the other hand, SAM decreased the excretory quantity of PEB, but it did not affect its excretory rate. Decomposition of AMB and PEB in the supernatant fraction of liver after centrifugation at 9,000 x g was markedly inhibited by SAM. Binding of AMB and PEB to bovine serum albumin was also inhibited by the administration of SAM. The administration of SAM did not have any effect on their partition coefficients.