The biological disposition of GPA 1851, a compound with anti-inflammatory proper ties, has been investigated in several species. During these studies a dramatic prolongation of barbitu rate sleeping time was observed in rats, dogs and monkeys pretreated with GPA 1851. Both in vivo and in vitro findings indicated that the interaction of GPA 1851 with barbiturates was a result of its inhibition of hepatic microsomal enzyme activity. However, further studies demonstrated that repeated dosing with GPA 1851 abolished the inhibitory effect of a single dose.