The proposed experiments study the measurement of time by animals. The times involved are durations on the order of seconds; the subjects are rats. In a situation where rats press a lever for food, they are trained to respond differently after different durations (e.g., press one lever after a 1-sec light and another lever after a 4-sec light). Efficient performance on this task reveals the existence of some sort of internal clock. The proposed work uses variations of this task to find out properties of the clock. The major questions are: (a) What stimuli start the clock? (b) Is the clock used for duration discrimination the same or different than other internal clocks? (c) Is the rate of the clock affected by the stimulus being timed?