A physically-based model has been developed for certain photoresponsive cells in the Aplysia californica central ganglia. The R2 giant neuron and the ventral photoresponsive neuron hyperpolarize when illuminated, due to an increase of membrane permeability to potassium. We hypothesize that light releases an internal transmitter from cytoplasmic granules. Three model compartments parallel cellular morphology: the first represents the granule component with bulk-limited diffusion; the second corresponds to cytoplasm and involves simple diffusion; and the third compartment, near the membrane, relates membrane conductance of potassium to the transmitter concentration adjacent to the membrane.