Three groups of rats were raised in environments of diurnal lighting, constant lighting and total darkness following 18-21 days of diurnal lighting. Those groups exposed to 20 and 21 days of diurnal lighting before being placed in total darkness (groups 20D and 21D) matured later than any of the other groups and had the greatest pineal weights and lowest ovarian weights. The light-reared group, and the group reared for 18 days in diurnal lighting prior to exposure to total darkness (group 18D), matured earlier than the diurnal group and had the lowest pineal weights, with ovarian weights 2nd only to the diurnal group. The findings confirm earlier studies and reveal that 20 days of exposure to diurnal lighting is necessary for subsequent total darkness to produce a delay in pubertal onset.