1. Receptive fields of perifoveal ganglion cells have been measured by determining threshold for eliciting a just detectable response using either concentric spot stimuli centred on the receptive field or small spot stimuli in different parts of the receptive field at various states of retinal adaptation and with stimuli selected to separate rod from cone function.2. Light-adaptation decreases the sensitivity, latency and duration of threshold responses throughout the receptive field of a ganglion cell.3. With all patterns of retinal stimulation and states of adaptation, threshold signals of the rods reach a ganglion cell later and those of the cones earlier than approximately 50 msec after a light stimulus.4. In the more dark-adapted retina threshold rod and cone signals can be transmitted to the brain by the same or by neighbouring ganglion cells but not simultaneously; in the light-adapted state only the cone signal is transmitted.