High thermosensitivity of early embryos controlled by mutation l(1)ts403 with disturbed heat-shock response was studied. Thermosensitivity was examined in early (0-1 h) and late (3.5-4.5 h) embryos obtained by reciprocal crosses and backcrosses. It was shown that mutation l(1)ts403 lacks maternal effect. In progeny of reciprocal crosses, early embryonic thermosensitivity was intermediate with regard to that of progeny obtained by interlinear crosses. In early embryos of Drosophila, zygotic genes are not expressed and heatshock protein synthesis is not induced. Based on this, it was proposed that the product of gene l(1)ts403, which affects early embryonic thermosensitivity, is transmitted both paternally and maternally and shows dosage effect.