The emergence of sex- and estrous cycle-related differences in the anorectic effect of fenfluramine, a serotonin (5-HT) agonist, prompted us to investigate whether these behavioral changes are mediated by estradiol. Rats were ovariectomized and housed in cages that permitted the analysis of feeding and locomotor activity via an automated, computerized system. Using a within-subjects design, we investigated the effects of 1 mg/kg d-fenfluramine and saline vehicle on food intake and wheel running in ovariectomized rats following estradiol benzoate (EB) and oil vehicle treatment. A cyclic regimen of EB treatment was used to mimic the changes in endogenous estradiol secretion over the rat's 4-day estrous cycle. The decrease in food intake following fenfluramine treatment was greater in EB-treated rats, relative to oil-treated rats. Fenfluramine also produced a small but significant decrease in wheel running in ovariectomized rats that was not modulated by EB treatment. Thus, EB's ability to increase the anorectic effect of this dose of fenfluramine appears behaviorally specific. Although the inhibition of food intake by fenfluramine is largely attributed to its ability to increase synaptic levels of 5-HT, additional research involving selective 5-HT receptor agonists and antagonists is necessary to determine whether estradiol interacts with the endogenous 5-HT system to control food intake in the female rat.