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1. A study was made of the effects of the venom of the brown widow spider (Latrodectus geometricus) on end-plates of the frog sartorius muscle. 2. The increase in the frequency of the minature end-plate potentials (m.e.p.p.s), elicited by the venom in normal-"Ca2+" Ringer solution, occurs in discrete volleys having a sharp onset and end. The frequency of the m.e.p.p.s is high (up to 300 sec-1) and relatively constant during the volley. 3. The volleys recur at intervals during a period from 5 to 10 min after addition of the venom until the onset of electrical silence, up to 4 hr later. The activity occurs in groups containing volleys of loing and short duration. 4. Simultaneous intracellular and extracellular recording from single end-plates indicates that the volleys originate at highly localized areas of the nerve terminals. The high-frequency release of m.e.p.p.s in hypertonic sol solutions, which was studied for comparison purposes, occurs randomly over the entire end-plate. Volleys originating simultaneously at different sites are often superimposed in the intracellular recordings. 5. In high-"Ca2+" Ringer solution, the initial frequency of the m.e.p.p.s in a volley is comparatively higher. However, the frequency drops to one half its value in a few seconds. The volley then terminates or else the frequency of m.e.p.p.s remains high for some time and the volley has no sharp end. Activity occurs in groups containing both long and short volleys. Many more short (less than 5 sec) and long (greater than 30 sec) volleys occur in high-"Ca2+" solution than in normal-"Ca2+" solutions. 6. In low-"Ca2+", high-"Mg2+" Ringer solution, the volleys of m.e.p.p.s are fewer in number and much longer in duration. Intra- and extra-cellular recording of uninterruped activity during long periods suggests that in this solution the m.e.p.p.s originate diffusely rather than at discrete areas of the nerve terminals. 7. Implications of the above data on possible modes of action of the venom are discussed.

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