Little information is available on the contribution of the isomers of pentobarbital and secobarbital to the behavioral effects of the racemic mixtures. To determine the contribution of each isomer on food-maintained behavior, pigeons were trained to respond under either a multiple fixed-ratio 30, fixed-interval 600 (mult FR30 FI600) schedule, or a multiple fixed-ratio 30, variable-interval 60-sec (mult FR30 VI60) schedule under which every response under both the FR and VI components also produced a brief electrical shock (punishment). Dose-response curves were determined separately for each isomer of pentobarbital (1-17.5 mg/kg i.m.) and secobarbital (1-17.5 mg/kg i.m.). Under the mult FR30 FI600 schedule, the S-(-)-isomers of pentobarbital and secobarbital were slightly less than 2-fold more potent than the R-(+)-isomers, but both isomers produced qualitatively similar effects on responding. The S-(-)-isomers of both barbiturates produced increases in punished responding under the mult FR30 VI60 at lower doses than the R-(+)-isomers. At high doses (greater than 10 mg/kg), the S-(-)-isomers suppressed responding early in the session resulting in a lower average rate of punished responding for the session than the R-(+)-isomers. The peak increase in punished responding was observed after 10 mg/kg of the R-(+)-isomers of pentobarbital and secobarbital. These results indicate that the effect of the isomers of pentobarbital and secobarbital on mult FR30 FI600 responding and on suppressed responding are qualitatively similar.(ABSTRACT TRUNCATED AT 250 WORDS)