Dark and light adapted ERGs were recorded in 19 patients with XLRS and in 15 patients with CSNB. Patients were assigned to clinical groups after identification of mutations in the RS1 (16 patients), NYX (11 patients) and CACNA1F (4 patients) genes causing XLRS, 'complete' CSNB and 'incomplete' CSNB, respectively. ERG responses were compared with those of 26 healthy volunteers. Rod responses were most severely affected in patients in the NYX group but a rod-generated b-wave could be identified in the majority of patients in this group. Rod responses were less severely affected in the CACNA1F and RS1 groups and ERGs did not differ significantly between these two groups. Cone reponses were largely unaffected in the NYX group but were abnormal in the RS1 and especially CACNA1F groups. The ERG results suggest that the RS1 and CACNA1F gene products have comparable functional consequences and that all three genes may affect multiple retinal sites.