The onset of lay, quality of eggs during early lay and gonadal development of both sexes were analysed in the HG and LG lines divergently selected for high and low relative weight gain between 11 and 28 d of age, respectively, and constant BW at 49 d ofage. The LG line was sexually mature at an earlier age and lower BW than the HG line. This corresponded with the trend during embryonic and early postnatal development. Analysis of gonads also confirmed earlier sexual development in the LG than in the HGline. Despite the different age and BW at onset of lay, HG and LG quail commenced lay at the same degree of maturity – 90% of adult BW. Thus, the onset of sexual maturity could be identified as a point on the growth curve which terminates the linear growth phase. When compared with the LG line, the HG line was characterised by a longer acceleration and shorter retardation growth phase. This difference is seen as an important determinant of line differences in growth and reproductive per