Males of the mouth breeding cichlid fish H. burtoni were visually isolated from conspecifics for periods of 15 min-12 days. The number of attacks directed at adult conspecifics increased by 2 and 7 h of isolation, but after sufficiently long isolation (12 days) the attack rate decreased. The sexual activity increased after only 15 min of isolation. With longer periods of isolation this increase became more pronounced. The aggressive and sexual behaviors were differently affected by isolation; this disputes any influence on reproductive behavior in general. Shortly after termination of isolation the sexual activity was generally high while the attack rate was low. Throughout the first 30 min of contact with conspecifics the attack rate increased while the sexual activity decreased. After the longest periods of isolation the maximal sexual activity tended to occur later (5-15 min after the isolation). After isolation periods of 3 and 12 days the increase of the sexual activity lasted a couple of hours while the decrease in the attack rate after 12 days of isolation seemed to persist for several days. The results can be explained in terms of adaptive short-term and long-term incremental and decremental processes in different motivational systems, and an attempt is made to relate the results to the biological conditions of the species.