Thirty‐two southern Swedish families ofPinus sylvestrisL. from a partial 11×11 factorial mating were grown at four night lengths, 3, 5, 7, and 9 h, from the start of their first growth period. At the end of the growth period, the plants were exposed to increasingly longer nights until total growth cessation. During the second and third growth periods, the plants shared the same growing conditions in the growth chambers, starting with two weeks of 6‐h night followed by night prolongation as in the first growth period. The plants from the 9‐h treatment grew more slowly than those from the other treatments in growth periods 2 and 3. The genetic influence of the growing conditions during the first growth period on the height increment during growth periods 2 and 3 was estimated by the female and male by treatment interaction variances. These variance components were relatively small and totally negligible when data from the 9‐h treatment were excluded from the analysis. The data suggest stability in the parents with respect to preconditioning caused by varying photoperiods.