In Drosophila montana, the male courtship song (especially the pulse characters of the song) plays an important role in sexual selection. The heritabilities and the amount of additive and residual variation in different characters of the male song were measured in two populations using father-son regression and sib analysis. The songs of the males from the Oulanka population were recorded for a second time after the males had been kept in 4 degrees C for 6 months. Heritabilities measured for different song traits were non-significant in each case, largely due to high residual variation. During the cold treatment, the additive variation increased and the residual variation decreased in nearly all song traits. Our results suggest that genotype-environment interactions may increase the amount of additive variation between males in sexually selected song traits during overwintering, prior to the mating season of the flies.