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The effect of nutrition on gonadotrophin secretion may be exerted through a central metabolic signal that reflects nutritional status. We have previously found that glucose and insulin concentrations are elevated in the cerebrospinal fluid (CSF) of rams in which the secretion of gonadotrophins has been stimulated by a nutritional supplement of lupin grain ( Lupinus angustifolius ). In the present study, we tested the hypothesis that insulin and/or glucose is a metabolic modulator of GnRH secretion and mediates the effects of nutrition on gonadotrophin secretion. Six mature rams were fed a diet that maintained live weight and then given a series of infusions, each for 12 h/day for 4 days, in a cross-over design. The treatments were: artificial CSF (aCSF), glucose (50 μmol/h) in aCSF, insulin (0·6 ng/h) in aCSF, and glucose (50 μmol/h) plus insulin (0·6 ng/h) in aCSF; all infused at a rate of 5 μl/min …