Using one microelectrode inserted into a Limulus retinular cell, simultaneous recordings were made of the receptor potential evoked by light falling on that retinular cell and of the optic nerve action potentials generated in the eccentric cell coupled to that retinular cell. The postreceptor transfer function for sensory quantity was thereby examined directly between the retinular cell receptor potential and the eccentric cell action potentials over a more extended range than had been reported in previous studies. This transfer function exhibited a strong non-linearity; the function had 3 discrete linear segments. It is similar to the function relating extrinsic current to spike frequency in cat motoneuron. Light adaptation induced a shift of the transfer function. There were certain phenomenological similarities between the effect of light adaptation on the receptor and postreceptor transfer functions. But there were also enough dissimilarities to suggest that the mechanism of postreceptor adaptation is different than the receptor adaptation mechanism.