Experiments were conducted to determine the relationship between photoperiod cues and the modulatory action of octopamine (OA) on male sensitivity to sex pheromone in the cabbage looper moth. Results showed that levels of random locomotor activity and response to pheromone, as well as the effectiveness of OA in enhancing male sensitivity, were dependent on the light intensity during the scotophase period. The importance of scotophase onset was further demonstrated by the fact that a 1 h period of dark (instead of the normal 8) was sufficient to elicit male response to pheromone, and the modulatory action of the amine, several hours later in photophase conditions during the “expected” mid-scotophase period when peak response normally occurs. Furthermore, expression of the response rhythms and the effect of octopamine were maximal when injection of the amine and scotophase onset occurred during a narrow temporal window around the expected time of scotophase onset. Delays or advances in scotophase onset or injection of octopamine resulted in decreased levels of behavior and loss of effectiveness of the amine. Studies involving a reversal in the photoperiod also support the idea that scotophase onset is a critical time in the regulation of the general locomotor and pheromone-response rhythms exhibited by males. The combined results support a hypothesis that the modulatory action of octopamine is associated with physiological changes occurring at the onset of scotophase, and that the periodicity of this action is influenced by an endogenous oscillator. © 1992 Wiley-Liss, Inc.