Pheromones trigger reproductive responses of many marine organisms, but little is known about how pheromones mediate mate-finding behavior in the marine environment. This paper investigates whether the tetrapeptide nereithione (cysteine-glutathione disulfide), known to be released by females of the polychaete Nereis succinea to trigger spawning in male N. succinea, can also be used at lower concentrations to guide males to the females. Low concentrations of pheromone elicited increased swim speed and turning left or right 84% of the time. Animals sometimes weaved back and forth, or in other cases swam straight along the trails an average of 8.1+/-1.2 cm before veering off. At higher concentrations, the males circled frequently, often encountering 10-20 cm of pheromone trail before swimming away. Male responses to nereithione were modeled by computer simulation, taking into account arousal of swim speed, activation of turning, speed of response and its decay, etc. In the model, low concentrations (<10(-8) mol l(-1)) of pheromone significantly increased the number of encounters with the pheromone trail, an average following of simulated trails of 10.5+/-3.6 cm, and a significant increase in the frequency of encountering a virtual female on the trail (ANOVA, P<0.001). The model supports the hypothesis that a pheromone can have a dual function, with low concentration pheromone trails being used by male N. succinea to find females and increase their likelihood of mating whereas high concentrations of the same pheromone trigger the spawning behavior itself.