On the mechanism of the interaction of ketamine and halothane in vitro.

1. Electrically induced contraction of guinea pig ileum myenteric plexus-longitudinal muscle was inhibited by ketamine and halothane with IC50s of 2.1 x 10(-4) M and 1.8 v/v% respectively. 2. The inhibitory action of ketamine was partially antagonized by naloxone and the selective kappa antagonist nor-binaltorphimine. 3. The actions of ketamine and halothane were synergistic at high levels of response (above 30% inhibition). 4. The actions of ketamine and halothane became antagonistic after treatment with pertussis toxin. 5. The interaction of ketamine and halothane was similar to the interaction of morphine and halothane.