1. The effect of curare on the amount of transmitter released by a nerve stimulus was studied in frog and rat nerve-muscle preparations using electrophysiological techniques.2. When the frog sartorius nerve-muscle preparation was exposed to low doses of curare, the amplitudes of spontaneous miniature end-plate potentials and end-plate currents (e.p.c.s, measured under ;voltage clamp' conditions) were reduced to the same extent, suggesting that the drug did not alter the number of transmitter quanta released by nerve stimulation.3. With higher doses of curare in frog muscle treated with glycerol to abolish twitching, quantum content was estimated from the coefficient of variation (CV) of e.p.c.s. The measured CV increased slightly in curare; this increase probably resulted from a relatively greater contribution of random noise to the observed fluctuations when the e.p.c. was reduced by curare.4. In the rat diaphragm, muscle fibres were cut to block twitching. This procedure produced, among other changes, a reduction in the muscle fibre space constant, so that junctional signals were distorted by the cable properties of the muscle fibre when, as often occurred, micro-electrodes were more than 100-200 mu from the end-plate focus. This produced errors in estimates of quantum content; when these errors were accounted for, it appeared that curare did not significantly alter quantum content.5. It is concluded that if curare affects transmitter release at all, its effect must be much smaller than its well known post-synaptic blocking action.