Dark-adapted retinae of the toad and bullfrog were found to respond to brief light stimuli with a succession of rapid mechanical changes. The latencies of the mechanical responses, as well as the effects of chemicals known to block the synapses on photoreceptor cells, indicate that the first mechanical response represents swelling of the photoreceptor cells. The first response is followed by mechanical changes in the postsynaptic elements. It is suggested that the observed response of the photoreceptor cells is a mechanical expression of the process underlying heat production by the cells.