We describe the cannulated Rana pipiens, tadpole preparation that allows for stable recording in the tectum of the extracellular potential elicited by optic nerve stimulation. The largest components of the evoked tectal response consist of two previously identified waves and a major third, long-latency wave of long duration. These components were reversibly eliminated by perfusion of high magnesium/no calcium Ringer's solution or Ringer's solution containing cobalt chloride. In contrast, perfusion of high calcium/no magnesium Ringer's increased the amplitude and area of these components. We conclude that these components represent post-synaptic activity. Additionally, small, short-duration waves were identified as arising from the activity of retinal afferents. They consisted of a short-latency (3.1-7.6 ms) and a long-latency (12-23 ms) group. Waves belonging to both of these classes were still visible in both high magnesium/no calcium Ringer's solution or Ringer's solution containing cobalt chloride and were unaffected by high calcium/no magnesium Ringer's. The average conduction velocities of the short- and long-latency groups matched the conduction velocities of, respectively, edge and convexity detectors in the adult. This indicates that retinal afferent input may already be present in adult patterns at the time that tectal circuitry is developing.