The addition of lupin grain to a maintenance diet increases circulating concentrations of volatile fatty acids and gonadotrophin secretion in mature rams. The experiments reported in this paper tested whether these responses were linked causally. The first experiment used 24 rams, of which 16 had two intra-ruminal cannulae inserted each, one for infusion of buffer and the other for infusion of volatile fatty acids. The treated rams (n = 8) were infused for 10 h each day with 1 litre of a solution of volatile fatty acids containing 1.93 moles acetate, 1.89 moles propionate and 0.68 moles butyrate, and 2 litres of a buffer solution containing 14.78 moles sodium bicarbonate, 6.39 moles potassium bicarbonate and 2.06 moles sodium chloride. The eight negative control rams were fed to maintenance and four of them were infused intra-ruminally each day with 3 litres of buffer containing the same mass of salts as the treated rams were given. The eight positive control rams were fed the maintenance diet plus a daily supplement of 900 g lupin grain and four of them were also infused with buffer salts. Blood was sampled before treatment, on Day 3 and at the end of the treatment period (Day 10) for measurement of the plasma concentrations of gonadotrophins (LH and FSH), glucose, insulin and cortisol. In the second experiment, another 24 mature rams were fed to maintain constant live weight and, for 12 of them, sodium acetate (127 g), sodium propionate (64 g) and vegetable oil (60 g) were added to the diet every day for 11 days. Blood was sampled before, and at the end of the treatment period for measurement of the plasma concentrations of gonadotrophins, inhibin, glucose and insulin. In Experiment 1, intraruminal infusion of buffer alone did not affect any of the endocrine variables measured. The infusion of fatty acids and buffer did not alter gonadotrophin secretion, but depressed food intake and increased the plasma concentrations of glucose, insulin and cortisol. When the fatty acids were added to the diet (Experiment 2), the pulsatile secretion of LH was stimulated and FSH and inhibin concentrations were increased. Lupin feeding increased LH pulse frequency on Days 5 and 10, but did not affect FSH concentrations. There were no changes in the circulating concentrations of glucose or insulin. The effects of intra-ruminal infusion of fatty acids on cortisol concentrations and appetite, which were completely absent when similar amounts of fatty acids were fed, suggest that such infusions have undesirable, perhaps stressful, side-effects that prevent expression of the reproductive endocrine responses. On the other hand, the stimulation of gonadotrophin secretion following addition of fatty acids to the diet possibly suggests that changes in the plane of nutrition of rams, for example with lupin supplements, stimulates the reproductive centres of the brain via pathways associated with the synthesis or utilisation of fatty acids. These pathways do not seem to involve changes in glucose homeostasis. (C) 1997 Elsevier Science B.V.