Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes that regulates food intake and energy expenditure. Growth hormone (GH) secretion is markedly influenced by body weight being markedly suppressed in obesity and underweight. The aim of the present study was to study whether leptin can act as a metabolic signal connecting the adipose tissue with the growth hormone axis. We administered leptin antiserum (10 ul, i.c.v.) or normal rabbit serum (NRS; 10 ul, i.c.v.) to freely moving fed rats. Furthermore we assessed the effect of leptin administration (10 ug, i.c.v.) on fed and fasted rats. Spontaneous GH secretion was assessed over 6 hours with blood samples taken every 15 min. Administration of leptin antiserum led to a decrease in spontaneous GH secretion as assessed by the area under the curve (AUC) (168 ± 72 ng/ml/6h) in comparison to NRS-treated rats (813 ± 179 ng/ml/6 h, p < 0.01). While leptin administration (10 ug/rat; i.c.v.) to normal fed rats did not modify spontaneous GH secretion, leptin administration to fasted rats led to a reversal of the inhibitory effect exerted by fasting on GH secretion (AUC, 1650 ± 351 ng/ml.6 h vs77 ± 32 ng/ml/6 h, p < 0.01). This data suggests that leptin is a metabolic signal that regulates GH secretion.