We tested the hypothesis that improved availability of energy through carbohydrate ingestion could counteract the documented suppression of pituitary-gonadal function during prolonged exercise. Nine trained males repeated twice a 36 kilometre running exercise with two weeks interval. During the tests the subjects ingested in a randomized single-blind fashion a total of 1050 millilitres of carbohydrate and placebo solutions. The total amount of ingested carbohydrate was 105 grams. Venous blood samples were taken before the exercise (sample A), immediately after exercise (B), and 2 hours later (C). In the B samples plasma glucose was 14% higher (P less than 0.01) and serum cortisol 13% higher (P less than 0.05) in the carbohydrate than in the control trial. In contrast, the level of LH was 18% (P less than 0.05) and that of FSH 11% lower (P less than 0.05) in the carbohydrate than in the placebo trial. Serum testosterone concentration did not differ between the treatment groups. We conclude that carbohydrate ingestion does not counteract the exercise-associated suppression of gonadotrophin secretion, but results in paradoxical increase of serum cortisol and decrease of LH and FSH, in comparison to placebo-treated controls. The higher cortisol level in carbohydrate group may be secondary to higher insulin levels.