Can rats discriminate between short (i.e., 10 s) and long (i.e., 50 s) intervals of opportunity to run in running wheels? Eight female Long-Evans rats were exposed to 50 opportunities to run in a wheel. Following each interval, rats made a response to identify the interval. A correct response produced 0.1 ml of 15% sucrose solution, while an incorrect response caused the chamber lights to flash on and off for 5 s. Five of 8 rats met the criterion of 5 successive sessions with 80% correct responses for both durations. Furthermore, the criterion of 80% correct responses in a session was met more often with the 10-s than with the 50-s duration. Rats can discriminate between opportunities to run for different durations, but the results cast doubt on the assumption that rats are timing the intervals.