To test the hypothesis that function of the rod photoreceptors is abnormal in pediatric patients with mitochondrial disorders. Methods: Patients (n = 22; median age, 5 years) with a deficiency of 1 or more of the mitochondrial enzyme complexes, or a mutation in mitochondrial DNA, were studied by means of scotopic, full-field electroretinography (ERG). The conditions of ERG testing allowed derivation of the parameters of the activation of rod phototransduction from the ERG a-wave, and postreceptoral function from b-wave and P 2 stimulus-response functions. The deactivation of phototransduction was studied in 5 patients. The patients' ERG responses were compared with those of healthy control subjects (n=25). Results: Responses from 19 patients were sufficient for RESULTS Sample a-and b-wave results from 4 1 ⁄2year-old patient 22 are shown in Figure 1. Nineteen of the 22 patients had sufficient response amplitudes for rod photore-CLINICAL SCIENCES