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1. In the nerve—muscle preparation, where catch was characteristically minimal, 5-hydroxytryptamine (5-HT) had no effect on resting membrane potential, junction potentials, spikes or contraction.

2. In muscle bundles, where catch was prominent, 5-HT did not change membrane potentials, but prolonged junction potentials and lowered the threshold for spike discharge and contraction.

3. In muscle bundles, exposed to high concentrations of 5-HT, depolarization evoked repetitive spikes, while in low 5-HT, spikes were seldom fired even with much greater depolarization.

4. In muscle bundles, the effective membrane resistance, R_eff., decreased from 45-60 to 23-35 M as 5-HT concentration was increased.

5. It is suggested that 5-HT may facilitate spike discharge by lowering the internal free Ca²⁺ concentration.