ATX II is a toxin extracted from tentacles of Anemonia sulcata. It was known that this protein displays neurotoxic effects on frog isolated neuromuscular preparation (Fig. 1, 2) and that muscular contractures observed with ATX II are blocked by d-tubocurarine (Fig. 3) or on a 40-days-denervated gastrocnemius (Fig. 4). Part of these experiments has already appeared. 1. These effects of ATX II depend on calcium concentration in the bathing medium, as is the case for transmitter release. The same results were observed when we substituted strontium to calcium. 2. On an intact sciatic sartorius preparation, ATX II does not act on the amplitude of the miniature endplate potentials (mepps, Fig. 6). The muscular action potential is not modified by this toxin. 3. ATX II increases the frequency of the mepps (Fig. 5). The evoked transmitter release (quantal content) after ATX II is also largely increased (Fig. 7). 4. In conclusion, it is suggested that ATX II acts indirectly on the muscle through an increase in acetylcholine release from the motor nerve terminals.