Removal of the superficial pineal gland prevents testicular regression from occurring in golden hamsters exposed to a short-day photoperiod, but to date no studies have examined the role of the deep pineal gland in the photoperiodic control of gonadal function. We investigated the role of the deep pineal gland in this process by monitoring testicular size in hamsters with an electrolytic lesion of the deep pineal gland. Sexually mature hamsters that had been sham-lesioned or that had received a lesion aimed at the deep pineal gland were transferred from 14L:10D to 6L:18D or were maintained in 14L:10D for 73 d. Testicular widths were determined at 2-3-wk intervals and after 73 d the testes were removed and weighed. The mean testis widths of the seven animals with histologically confirmed deep pineal lesions and exposed to 14L:10D remained large, similar to those of the sham-operated control animals exposed to the same photoperiod. The testes regressed in the five animals with confirmed deep pineal lesions and exposed to 6L:18D in a similar manner to those of the sham-operated control animals exposed to the same photoperiod. No significant difference (P greater than .05) was found between the testicular weights of lesioned and unlesioned animals exposed to either photoperiod. The lack of a significant effect of a deep pineal lesion on testicular size demonstrates that this portion of the pineal complex is not required for normal regulation of testicular function by photoperiod. These results also suggest that nerve fibres projecting from the brain to the superficial pineal gland via the deep pineal gland are not involved in photoperiodic regulation of testicular function.