These experiments tested the effect of 10 to 30 mg, citalopram/kg body weight on food intake, weight increase, and blood glucose levels in young obese hyperglycemic mice (Umeå ob/ob). A leptin defect in ob/ob mice results in hyperphagia, hyperglycemia, and increased body weight compared with normal mice. Citalopram had no effect on weight increase in ob/ob mice aged 3 to 10 weeks, when the weight increase is most rapid. Citalopram reduced the weight increase at the age 10 to 19 weeks. Food intake reaches a maximum at age 7 to 10 weeks and then decreases. The reduction was more rapid in citalopram-treated mice. The weight of feces paralleled the food intake. Citalopram treatment had no effect on serum insulin levels in 15-week-old mice. Blood sugar values in fed mice reached a peak at age 7 weeks (21.7 +/- 1.7 mmol/L in controls and 22.3 +/- 1 mmol/L in citalopram-treated mice). After that, blood sugar values decreased. The decrease was more pronounced in citalopram-treated mice (P < .01 compared with controls). Blood glucose levels were lower at ages 12 to 15 weeks in female ob/ob control mice (13.6 +/- 2.5 mmol/L v 19.0 +/- 0.6 mmol/L in male control mice; P < .05). The effect of citalopram was the same in male and female mice. There was a close correlation between accumulated food intake and blood glucose values in individual animals. At age 3 to 10 weeks, ob/ob mice have a high beta-cell proliferation rate, and they have large islets of Langerhans. This was not affected by citalopram treatment. Our findings show that the serotonergic system plays a role as a regulator of food intake over shorter periods, and this is also true in the absence of leptin.