Psychophysical and electrophysiological methods are used to evaluate retinal function in a family with dominantly inherited retinitis pigmentosa. Techniques of full-field (ganzfeld) stimulation, scotopically balanced lights, and a ganzfeld adapting field are introduced to analyze the electroretinogram (ERG). Rod ERG responses, reduced in amplitude and delayed in latency, are detectable in the early stages of the disease. Cone ERG responses are normal. Dark adaptation testing shows normal cone but abnormal rod function with a slight delay in the rate of rod recovery. Scotopic luminosity curves and ERG responses to scotopically matched lights suggest that the rhodopsin present has a normal absorption spectrum. A reduction in the amount or effectiveness of rhodopsin in each rod throughout the entire retina can explain the findings. The literature on light-induced retinal degenerations is reviewed, and the effects of light exclusion on this disease are considered.