1) Male three-spined sticklebacks which were clearly in the sexual phase of their reproductive cycle, were presented with aggression eliciting stimuli for a period of fifteen minutes each day for ten days. One group was stimulated with a live, nuptially colored male stickleback in a clear plastic tube each day. Another group was presented with a wooden model of a male stickleback each day. The third group received no aggression eliciting stimulation. Both groups which received stimuli showed a decrement in responding to the stimuli over the ten days; the more dramatic decrement was found with the group presented with a live stimulus since this stimulus elicited the greatest aggression initially. 2) Aggressive responses did not all wane at the same rate, but rather one, charges at the stimulus, dropped out first, followed by bites. Orientations toward the stimulus decreased over the ten days but never reached a low level. 3) All three groups were presented with a gravid female stimulus for five minutes each day. The group presented with a model of a male stickleback and the group presented with no male stimulus both showed no change in the level of sexual response directed toward the female stimulus. The group presented with the live male stimulus demonstrated an increasing number of sexual responses directed at the gravid female.