Pigeons were trained in two experiments that used the concurrent-chains procedure. These experiments sought to identify the variables controlling the preference of pigeons for a constant duration over a variable duration of exposure to an aperiodic, time-based, terminal-link schedule. The results indicated that two variables correlated with the constant-duration terminal link combined to control preference: (a) a shorter initial delay to a reinforcer; and (b) the probabilistic occurrence of multiple reinforcers. (Contains 2 tables and 9 figures.)