The prevalence of obesity in domestic cats is increasing worldwide, and is strongly associated with gonadectomy. We have previously demonstrated the effectiveness of oestradiol in reducing food intake in both male and female neutered cats. This experiment was designed to test the hypothesis that oestradiol or genistein would prevent the increase in food intake following gonadectomy of male and female cats, and would prevent an increase in body fat mass. Three groups of eight cats each were surgically neutered then treated daily with either 0.5 mug oestradiol subcutaneously, 100 mg/kg genistein orally, or vehicle only. Effect of treatment on food intake, vaginal cytology and body weight were recorded, and body composition was assayed using the D(2)O isotopic dilution method. Neutering was followed by an increase in food intake, bodyweight and body fat mass in the control group, which were almost completely prevented by treatment with oestradiol (p < 0.001). Treatment with genistein had no effect on food intake or bodyweight increase, but was associated with a significant increase in lean body mass (p = 0.018), and significantly less body fat accumulation than the control group (p = 0.01). There were no significant differences in responses to treatment between sexes. These findings demonstrate the importance of gonadal oestrogen for the control of food intake in male and female cats, and suggest the provision of an oestrogenic compound could help prevent obesity following neutering. In addition, the findings of this study are consistent with observations in rodents of the efficacy of genistein in inhibiting adipogenesis and promoting lean body tissue development.