Adult Charles River female rats were pinealectomized, sham-operated or left intact, and half of each group was placed in 12L:12D while the other half was exposed to constant light (LL). Daily vaginal smears permitted the calculation, at weekly intervals, of the percent of rats in each group in LL showing 5 or more consecutive days of vaginal cornification. After 11 weeks in LL, those rats were returned to LD for 6 weeks and then put back into LL until the termination of the experiment 3 weeks later. Pinealectomized, sham-operated and intact animals developed persistent estrus (PE) in LL at the same rate, began recycling at the same rate when returned to LD, and again went into PE at the same rate when put back into LL. Confirming our earlier observations, pinealectomized rats in 12L:12D showed significantly fewer 4-day cycles (or more 5-day cycles) than did intact rats in the same photoperiod. Body and organ weights and serum LH levels showed no differences attributable to pinealectomy. It is concluded that the pineal is not influential in the induction of PE in LL, but its apparent role in controlling cycle length in shorter photoperiods is confirmed.