The pheromone nereithione (cysteine-glutathione disulfide), which is released by swimming females of the polychaete Nereis succinea to activate spawning behavior of N. succinea males, has recently been identified and synthesized. Nereithione activates sperm release at less than 10(-6) M, one to two orders of magnitude less than oxidized glutathione or any other glutathione derivative tested. The glutathione fragment gamma-glu-cys inhibited sperm release. Nereithione aroused three components of the male nuptial behavior: circling, sperm release, and accelerated swimming. Electrophysiological activity elicited by nereithione near the sperm release site consisted of initial large spikes, cyclic bursting activity, and small spikes lasting up to a minute and was dose dependent, rapid, reversible, and repeatable. This preparation is an excellent model system for characterizing the receptors and functions of a marine pheromone.