Heterozygotes of X-linked retinitis pigmentosa were studied with full field rod and cone electroretinography and light adapted kinetic perimetry. Twelve parameters from the electroretinograms (ERGs) and two parameters from the kinetic visual fields of both eyes of 22 heterozygotes were measured and statistical comparisons made with results from female control subjects. Rod and cone ERG amplitude parameters were significantly lower and cone timing delayed in the heterozygotes. Most of the ERG parameters that were abnormal in measured value also showed significantly greater interocular differences compared with controls. Kinetic visual fields with both V-4e and I-4e test targets were smaller in heterozygotes than in controls. Only with the I-4e target, however, were interocular differences significantly larger in the heterozygotes. For the I-4e target and many of the ERG parameters, using the interocular difference in conjunction with the measured parameter value from a single eye significantly increased the efficacy of discrimination between heterozygotes and controls; for some ERG parameters, the interocular difference alone provided the best separation of the two groups.