These experiments were designed to study the effects of age of blinding on the circadian rhythm in pituitary-adrenal function in female rats. In the 1st experiment, the eyes were removed at 1 day of age and serial blood samples were obtained at 6-h intervals for 24 h at 45 days. In the 2nd experiment, rats were blinded at 1, 26 or 60 days of age, and serial blood samples were collected at 4-h intervals for 44 h when the rats were 84, 112 and 142 days of age. Intact and sham-operated rats served as controls. Plasma corticosterone (cpd B) concentrations, measured πuorometrically, were used to assess pituitary-adrenal function. Blinding disrupted cpd B patterns in all groups on all occasions. However, steroid patterns of individual rats suggested the presence of free-running 24-h rhythms. This suggestion was supported by the results of aligning the steroid peaks of individual blinded rats. This procedure yielded group patterns of steroid levels which were comparable to those of controls. The periodic stimuli available to the blinded rats from intact rats in the same and adjacent cages did not synchronize this rhythm. These results suggest that the eyes and their central projections are not essential after birth for the development and maintenance of rhythmic pituitary-adrenal function; these structures appear necessary, however, for synchronization with light-dark cycles