The determinants of infants' low scotopic visual sensitivity are controversial. Some interpret infants' scotopic vision as indicative of immature rod photoreceptor function while others attribute infantile sensitivities mainly to post receptoral immaturities. To date the rod photoreceptor sensitivity of human infants has not actually been measured. In the work reported herein, electroretinographic a-wave responses, which represent the rod photoresponse, were recorded from dark adapted 10-week old infants and adult control subjects. Rod isolated a-waves indicate that 10-week-old infants' rods are less sensitive than adults'. Thus, any explanation of infants' scotopic visual sensitivity must take into account this fundamental property of infants' rods, low sensitivity.