1. Electrical potentials associated with coordinated contraction of the longitudinal muscles can be recorded from isolated tentacles of Hydra. In both H. pirardi and H. pseudoligactis these Tentacle Contraction Pulses (TCP's) occur chiefly in bursts, which are more frequent in H. pirardi. The bursts are approximately the same length in the two species but those of H. pirardi contain more pulses. 2. Inter-pulse interval lengths within a burst decrease but then lengthen again toward the end of the burst. Small segments of a tentacle produce the same pulse patterns as whole tentacles, indicating that there are a number of potential pacemakers with similar properties dispersed within a tentacle. 3. Strong illumination increases the burst frequency and the number of pulses per burst. Pulses of mechanical stimuli induce single TCP's and inhibit TCP bursts. Isolated tentacles habituate to repeated mechanical stimuli. Exposure of the tentacle to live or aqueous extracts of Artemia nauplii, or 10-5 M GSH inhibits TCP bursts and monophasic pulses are induced which are associated with asymmetric writhing movements.