Energetic expenditure during spawning of male and female 1 -sea-winter Atlantic salmon, Salmo salar L., was measured. Before spawning, the somatic energy content per unit weight did not differ between the sexes. The testes content offat was 0.24, of carbohydrate 8.89, of water 1.21 and of ash 1.61 times as high as that of the ovaries. Just prior to spawning, mean gonadosomatic index (GSI; wet weight, %) in males was 4.36 and in females 20.26, and expressed as energy ratios (kJ, %) 4.47 and 27.75, respectively. During spawning the energy loss of male soma was higher (35.57%) than that of females (25.00%). This was a result of higher loss of fat in males than in females. Total energy cost of spawning in males and females was on average 51.8 and 51.4%, respectively.