Few controlled studies have been made of the possible mechanisms and physiological consequences of weight gain after cats have been neutered. In this study, six male and six female cats were gonadectomised and compared with five entire male and six entire female cats, before they were neutered and one and three months later. The neutered males gained significantly more weight (mean [SEM] per cent) than the entire males (30.2 [5.2] v 11.8 [2.3]) and the entire females gained 40.0 (7.3) v 16.1 (3.3) per cent, (P < 0.05). The castrated males gained more weight as fat than the sexually intact males (22.0 [3.3] v 8.8 [4.5] per cent, P < 0.05). There was a significant increase (P < 0.05) in daily food intake after neutering. Spayed females underwent a significant decrease in fasting metabolic rate (83.7 [5.5] v 67.2 [2.3] kcal/kg bodyweight0.75/day P < 0.05). Gonadectomy had minimal effects on serum thyroid hormone concentrations, the resting or fasting metabolic rates in males, or on indices of glucose tolerance.