In rat an intravenous application (i.v.) of a lethal dose of the equinatoxin II (EqT II) provoked a respiratory arrest. It is not known whether the respiratory arrest is a result of a direct actions of the toxin on lung tissue, on neuromuscular junctions or the on the central nervous system. The influences of the toxin on the neuromuscular transmission and on muscular contraction were studied in isolated rat diaphragm. The effect of EqT II on end plate potentials of m. cutaneus pectoris was measured in the frog Rana esculenta. To monitor equinatoxin II effects on the central nervous system of a rat, the toxin was injected directly into the forth brain ventricle. The respiratory arrest was not the result of the toxin action on the neuromuscular junctions and peripheral nerves. The cessation of respiratory activity was most probably a result of equinatoxin II direct actions on lung tissue and on brain microcirculation.