1. The acidic peptide crotapotin potentiated the toxicity of the basic crotalus phospholipase A in all species tested. The order of sensitivity to the lethal action of the phospholipase was: chick greater than mouse larger than or equal to rabbit greater than rat. After a latency period of a least 20 min the animals died of respiratory paralysis. Rabbits survived for more than 10 h, if they were artificially respired. The animals recovered very slowly from respiratory depression. 2. In rabbits even high doses of the basic crotalus phospholipase A or its complex with crotapotin did not affect the respiratory center nor its reactivity to asphyxia. Conduction of action potentials in the phrenic nerve was not changed. 3. Phospholipase-crotapotin complexes decreased the contractile response of isolated phrenic-hemidiaphragms of rats to direct and indirect stimulation in an irreversible manner. A latency period 20-100 min preceded the paralysis. 4. A similar block of neuromuscular transmission developed in vivo. After i. v. injection of PC-complexes the contractile response of isolated rat phrenic diaphragms to nerve stimulation was considerably lower than to direct stimulation. 5. Crotalus phospholipase A alone as well as its complex with crotapotin reduced the contractions of the isolated chick biventer cervicis muscle but did not cause a contracture, thus indicating that the phospholiphase PC-complexes act not as depolarizing blockers. 6. Blood pressure, heart rate and electrocardiogram were not substantially altered, when phospholipase A or PC-complex were injected slowly i.v. into rabbits or rats. No cardiotoxic effect was observed in the Langendorff preparation of rat hearts perfused with phospholipase A alone (6 x 10(-6) M) or together with crotapotin (10(-5) M). Rapid i.v. injection of the venom produced hypotension. The degree of haemolysis correlated with the enzymatic activity, and was low for the highly toxic PC-complexes.