Bothropstoxin, a 13,700 mol. wt myotoxic phospholipase homologue isolated from the venom of Bothrops jararacussu and devoid of PLA2, proteolytic or hemolytic activities, inhibited muscle twitch tension, evoked either directly or indirectly through stimulation of the motor nerve in the mouse phrenic-diaphragm preparations. The compound action potential of the muscle was also abolished with a similar time course. In addition, the toxin (0.7 mM) evoked membrane depolarization which was inhibited in the presence of 10 mM Ca2+. In chick biventer cervicis muscle, the toxin (2 mM) induced a contracture that reached its maximum amplitude in 44.8 +/- 15.6 min (n = 6) and was not blocked by either d-tubocurarine or tetrodotoxin. The time to maximum amplitude was reduced to 5.5 +/- 1.0 min (n = 4) in nominally Ca(2+)-free Krebs solution and was completely abolished in Ca(2+)-free Krebs solution containing 1 mM EGTA.