Circadian rhythm in the pineal organ of the yellow-bellied toad, Bombina variegata (L.), under laboratory conditions.

A circadian morphological rhythm of the pineal organ, as judged by changes in the nuclear volume of the photoreceptor cells, was found in sexually mature male Yellow-bellied toads (Bombina variegata) maintained continuously at 26 degrees C and on a 17L/7D illumination cycle. The greatest nuclear volume occurred at the photophase (16.00 h), and the lowest at the end of the scotophase (4.00 h).