Ovariectomized rats, chronically treated with cannabis extract or control solution, were given different hormonal treatments. Results indicated that both cannabis-treated and estrogen-treated animals were more aggressive than controls. Furthermore, aggressiveness was virtually abolished when cannabis-treated females were made sexually receptive by estrogen and progesterone treatments. After 25 days of cannabis or control solution treatment, all subjects were sacrificed. The levels and turnover rate of brain 5-HT and peripheral plasma corticosterone were then assayed. Data indicated both a significant inverse relationship between plasma corticosterone and whole brain levels of 5-HT(r = -0.742 to -0.985) for all groups and a significant positive relationship between aggressive behavior and plasma corticosterone (r = +0.675 to +0.946) in all groups that were fighting prior to decapitation. Results are tentatively explained, suggesting that the variability of the female response to stress during the different phases of the estrus cycle, permitted them to perform differently after chronic cannabis treatment. 5-HT is apparently involved, either directly in its effects on aggressive behavior or indirectly through the pituitary-adrenocortical axis activation.