We studied fatigue of rat diaphragm in response to repetitive brief and prolonged electrical stimulation of the phrenic nerve, at 0.2, 1-100 Hz. Low and high frequency of stimulation produced twitch and tetanic contractions in the rat diaphragm. A mean maximum twitch tension of 1.4 +/- 0.1 g was produced at 1 Hz, and a mean maximum tetanic tension of 5.6 +/- 0.3 g was obtained at 100 Hz (means +/- S.E., n = 8). Twitch and tetanic fatigue was produced at all frequencies of stimulations, but with different time scale, or duration, and with different number of stimuli delivered to the muscle. At low rates of stimulation, e.g. 10 Hz, fewer stimuli were needed to fatigue the muscle (3000 in 5 min), whereas at high rates of stimulation, e.g. 50 Hz, more stimuli were needed to fatigue the muscle (6600 in 2.2 min). The amplitude of the tetanic tensions elicited at 10 and 50 Hz, at the end of 5 or 2 min fatiguing stimulation, was 39 +/- 2.7% and 80 +/- 3.1% of their respective control tensions (2.8 +/- 0 2 g and 5.3 +/- 0.5 g, n = 8, P 0.001). It was concluded that fatigue in the rat diaphragm depended on the frequency and duration of stimulation as well as on the number of stimuli delivered to the muscle. Various mechanisms of muscle fatigue are described in the discussion to explain the observations made in the present investigation.