The influence of sex hormones on the development of left ventricular hypertrophy was investigated in baroreceptor-denervated rats. A significant increase (p less than 0.01) in the left ventricular weight/body weight ratio was observed in male but not in female rats 15 days after operation, compared to age- and sex-matched sham-operated rats. This differential hypertrophy occurred despite the development of a significant elevation in arterial blood pressure in both sexes. Castration prior to sinoaortic denervation did not change the level of arterial hypertension but caused a significant reduction (p less than 0.01) in left ventricular weight in male rats and a significant increase (p less than 0.01) in female rats. The pretreatment of male and female sinoaortic denervated and castrated rats with testosterone resulted in ventricular hypertrophy similar to that observed in intact male sinoaortic denervated rats. Pretreatment with estradiol, however, suppressed the left ventricular hypertrophy in intact male rats but did not change the normal ventricular mass observed in intact female sinoaortic denervated rats. These results indicate that the development of left ventricular hypertrophy in sinoaortic denervated rats is modulated by sex hormones, and that testosterone exerts a facilitatory and estradiol an inhibitory action.