The impact of photoreceptor loss on the circadian system was examined by utilizing a transgenic mouse model (rdta) in which rod photoreceptors were specifically ablated. These mice were able to phase-shift their circadian locomotor behaviour in response to light, but features of this circadian behaviour were markedly altered. The amplitude of circadian responses to light were approximately 2.5 greater, the circadian period (tau) was reduced (c. 20 min) and the total duration of activity (alpha) was increased (c. 50 min) when compared to wild type (+/+) and rd/rd mice (retinal degeneration, mice which also lack rod photoreceptors) of the same genetic background. The pattern of Fos expression in the suprachiasmatic nuclei (the site of the primary circadian clock in mammals) was indistinguishable between +/+ and rdta mice. However, Fos expression in the retina suggested that rod loss in rdta mice resulted in a functional reorganization of the retina and the constitutive activation of a population of retinal ganglion cells. Although it has been known for several years that the entraining photoreceptors of mammals are ocular, and that rod photoreceptors are not required for light regulation of the clock, these are the first data to show that features of the circadian phenotype (amplitude of the phase response curve, alpha, tau) can be influenced by photoreceptor ablation. These data support the hypothesis that the circadian phenotype of mammals is the product of an interaction between the suprachiasmatic nuclei and the retina. Thus, mammals which show an altered circadian behaviour can no longer be assumed to have defects associated only with specific clock genes; genes that affect photoreceptor survival may also modify circadian behaviour.