Three stimuli: (A) darkness, (B) moderate light, and (C) light as much lighter than B as B was lighter than A, were presented to the subjects as illuminated tunnels in a discrimination box. Five [female] albino rats were presented with A and B, and trained to respond to B, when they were then transferred to B and C (A eliminated). The results of the transfers (12) showed that the rats had not learned to respond to B as an absolute stimulus and that they did not respond to C as a perfect equivalent of B but sufficiently positively to conclude that B had been known as a relative ("lighter than") stimulus.