Although delayed, vaginal opening occurred in all underfed groups. Two or three extended estrous cycles occurred after vaginal opening, then the animals became anestrus; however, cycling was restored when they were placed in constant light or with a male. Despite irregular estrous cycles in underfed females, they copulated, became pregnant and gave birth to litters of normal size and composition. Similarly, underfed males impregnated fed females. Continuous illumination or pinealectomy stimulated compensatory hypertrophy in the remaining ovary after hemiovariectomy both in rats underfed from birth and in starved adults which had achieved normal growth prior to underfeeding; however, the latter treatment was not as effective as the former. It did, however, increase ovarian weight prior to surgery. The adverse effects of underfeeding on reproductive function may be due to physiologic alteration resulting in a new pattern of control, rather than simply a reduced rate or lack of function due to less available energy, since the gonads may be stimulated to renewed activity by environmental or surgical manipulations.