PURPOSE: To assess temporal summation in children with a history of retinopathy of prematurity (ROP) by determining the critical duration (tCRIT) for complete temporal summation under rod-mediated conditions. From prior ERG studies, it is known that the kinetics of activation of phototransduction are prolonged in the ROP rod photoreceptor. METHODS: Dark-adapted thresholds for detecting 10° diameter stimuli with durations from 10 to 640 ms were measured. A two-alternative, spatial, forced-choice psychophysical procedure was used. The tCRIT for complete summation was estimated in former preterm subjects with a history of severe ROP (n = 7), mild ROP (n = 23), and no ROP (n = 15). The subjects ranged in age from 10.4 to 17.6 (median 15.6) years. Age-similar term-born control subjects (n = 5) were also tested. RESULTS: Critical duration was significantly longer in subjects with a history of ROP than in subjects who never had ROP or who were born at term. Mean tCRIT in the mild ROP group [127.5 (SD = 19.9) ms] and severe group [147.6 (SD = 18.9) ms] did not differ significantly, but both were significantly longer than in former preterms who never had ROP [101.1 (SD = 16.5) ms] and in term-born controls [101.0 (SD = 19.5) ms]. CONCLUSIONS: In ROP subjects, tCRIT is significantly prolonged. This is likely due to abnormal kinetics in the rod outer segment.