Fig. 1. Ketamine, at concentrations of 0.1–1 μm, significantly inhibited inspiratory frequency ( top ). At higher concentrations, ketamine either severely reduced (1 μm) or abolished (5 μm) inspiratory activity ( bottom ). The inhibition of respiratory activity by ketamine was gradually reversed after termination of ketamine exposure. In the absence of ketamine, respiratory frequency was well maintained for the duration of these experimental procedures (control data).