Tonic immobility (TI) is an unlearned behavioral response characterized by a state of immobility and torpor. Effect of inter-trial interval on duration of tonic immobility was assessed in thirty, juvenile lemon sharks (Negaprion brevirostris). Regression analyses showed that massed trials of 12 per session increased the average duration of tonic immobility by 475 sec compared to spaced trials of 1 per session. Each experiment was composed of 24 trials. TI is stable and durations much longer in the lemon shark than for other sharks. These findings have enabled us to develop a quantitative bioassay for use in testing chemical shark repellents.