Weight gain is a common problem in domestic cats, but little is known about its metabolic effects. The purpose of this study was to determine the effects of diet-induced weight gain and subsequent weight loss on metabolic rate, body composition, and glucose tolerance. Gain of approximately 20 per cent body weight (divided approximately equally between fat and fat-free mass) over three months resulted in insulin resistance in females, indicated by increases in basal insulin concentration (68·2 ±7·9 to 119 ±16·5 pmol litre-1, P<0·05), insulin peak response to glucose (241·1 ±31·6 to 315·0 ±23·0 pmol litre-1, P<0·05), and ΔI/ΔG (14·2 ±2·6 to 18·1 ±1·3 pmol mmol-1, P<0·05) compared with pre-gain values. The same numerical trend was noted in male cats, however, changes were not significant (P>0·05). Alterations in serum lipids included significant (P<0·05) elevations in triglyceride concentrations in male cals and decreased β-lipoprotein concentrations in both genders. Weight loss over three months normalised basal insulin, insulin response to glucose, and serum triglyceride concentrations, and resulted in significant (P<0·05) decreases in serum concentrations of β- and preβ-lipoproteins. cholesterol, and triiodothyronine. Diet-induced weight gain of three months' duration, followed by three months' maintenance of increased body weight did not affect fasting or resting metabolic rate. Development and severity of impaired glucose tolerance, insulin resistance, and other changes may be affected by duration and possibly severity of weight gain.