If it is assumed that the excitable tissue immediately associated with a rod receptor may generate impulses when light quanta are absorbed, then it follows that there is an upper limit to the number of impulses generated by each rod of ∼1/τ, where τ is the refractory period of the excitable tissue. Two possible modes of excitation are investigated. In the first, analogous to a Geiger counter of dead time τ, it is assumed that quanta absorbed during the refractory period delay recovery for a further time τ and this leads to a situation in which an increase in stimulus reduces the output of the rod beyond saturation. In the second, where the dead time is not prolonged by absorption during the refractory period, the output approaches asymptotically to the limit of 1/ τ as the stimulus is increased. Psychophysical increment threshold curves cannot distinguish between the two models but it is shown that ERG results in monkey and man can be fitted using the invariant refractory period model if equilibrium bleaching is taken into account.