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5-Hydroxytryptamine has wide-ranging effects on a number of physiological systems. In addition the exogenous application of 5-hydroxytryptamine or 5-hydroxytryptamine type drugs to tissue bath preparations has revealed the presence of a large and ever growing population of 5-hydroxytryptamine receptors. The discovery that 5-hydroxytryptamine is released at the neuromuscular junction raises the question as to whether it has a physiological function and whether or not applying low concentrations of 5-hydroxytryptamine transmitter would have any significant effect on twitch response at different rates of stimulation. Previous studies have demonstrated both facilitatory and depressant effects upon twitch responses in a variety of animal models. Single twitch responses of the rat phrenic nerve diaphragm preparation at non-fatiguing (0.3 Hz) and tetanic rates of stimulation (50 Hz) were recorded under conditions of indirect (nerve) stimulation at 4 g resting tension. The preparation was incubated with 0.5, 1, 2, 4, 16 mumol litre-1 concentrations of 5-hydroxytryptamine. The results of our investigations demonstrate that while low concentrations of 5-hydroxytryptamine have no effect upon the single twitch response at resting rates of stimulation (0.3 Hz), the initial single twitch and the subsequent eighteen responses immediately following a tetanus are facilitated (P < 0.05). This work demonstrates that 5-hydroxytryptamine does have significant effects upon the neuromuscular junction under conditions of physiological compromise (following tetanus). These observations are the first step, therefore, in a series of studies aimed at identifying the resident 5-hydroxytryptamine receptors within the neuromuscular junction which will subsequently enable us to predict therapeutic effect or toxicity.