A variety of different lighting regimes have been reported to cause alterations in the length and number of synaptic ribbons within photoreceptor synaptic terminals. Whether these alterations occur in mammalian visual systems during a diurnal lighting cycle was tested using computer-assisted morphometrics. Adult guinea pigs were entrained to a 12:12/ligh:dark regime and subsequently killed at various times throughout the cycle. Representative samples from all quadrants of the eyes were processed for electron microscopy and the synaptic ribbons within the synaptic terminals of the three different types of photoreceptors in this retina (alpha and paranuclear rods, and cones) were analyzed with the aid of an image analysis computer (Zeiss IBAS) for their length and absolute number per terminal. Contrary to previously published reports on other species, the synaptic ribbons in all three terminal types exhibited no statistically significant change in either number per terminal or in length, throughout the 24 hr cycle. Computer-based three-dimensional reconstructions from serial thin sections revealed horseshoe-shaped synaptic ribbons with pleats in numerous planes conforming to the contours of the invaginating postsynaptic elements.