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1. The effect of quinine on tension development and membrane potentials of crab skeletal muscle was examined using strain gauges and intracellular electrodes.

2. In low concentrations (0·1-0·5 mM), quinine caused transient potentiation of twitch tension which then rapidly declined along with progressive depression of the tetanus. These actions are correlated with the decline of both action and resting potentials during quinine treatment.

3. In moderate concentrations (1-5 mM), quinine induced phasic contractures, but the attendant depolarization made the muscles refractory to stimulation and potassium activation, but not to caffeine activation.

4. Quinine did not induce contractures in depolarized muscle, which suggests that the action of quinine in inducing calcium release from the sarcoplasmic reticulum may be blocked by potassium depolarization, unlike the calcium-releasing action of caffeine. Quinine appeared to have no effect on the mechanical threshold of crab skeletal muscle fibres.

5. To explain its depression of contractility in crab muscle, it is suggested that quinine may deplete the calcium store of the sarcoplasmic reticulum, leading to extinction of the terminal stages of the excitation—contraction coupling process and loss of contractility.