The effect of isoflurane administration on diaphragmatic activity was investigated in six anaesthetized mechanically ventilated dogs. Diaphragmatic strength was assessed by measuring the transdiaphragmatic pressure (Pdi) generated during supramaximal stimulation of both cervical phrenic nerves at frequencies of 0.5, 10, 20, 50 and 100 Hz under partially isometric conditions at 1, 1.5 and 2 minimum alveolar anaesthetic concentrations (MAC), after maintaining 1 h of stable conditions. Pdi measurements were made at the start of the stimulation (initial) and at the end of a 2-s period (2-s). The force-frequency relationship was compared at each anaesthetic level. For single twitch (0.5 Hz) stimulation, the time constant of diaphragmatic relaxation was also assessed. The sequence of changing anaesthetic depth was altered in random fashion between animals. Pdi amplitude at single twitch stimulation was unchanged at the three anaesthetic concentrations. There was no significant difference in initial Pdi at various stimulus frequencies with increasing depth of isoflurane anaesthesia. In addition, no change in 2-s Pdi during low frequency stimulation (10 and 20 Hz) was noted during any of the three levels of anaesthesia. By contrast, 2-s Pdi with 50 Hz stimulation during 2 MAC isoflurane exposure decreased significantly below Pdi levels seen at 1 and 1.5 MAC (P < 0.01). Furthermore, 2-s Pdi at 100 Hz stimulation decreased significantly in a dose-dependent fashion. From these results, we conclude that isoflurane reduces diaphragmatic activity at higher stimulation frequencies of 50 and 100 Hz.