Sleep and wakefulness were studied polygraphically in two adult male ferrets between the ages of 3 months and 2.5 years. Nine 24-hour recordings were obtained on different light/dark schedules, and one animal was also monitored for 3 days following administration of the serotonin synthesis inhibitor, parachlorophenylalanine. Stage scoring was accomplished utilizing criteria similar to those used in the cat. Certain modifications to the criteria were made to accommodate apparent differences in electrographic indicators of state. Mean daily percentages [+/- standard error of the mean (SEM)] for the major stages were: slow-wave sleep, 36.0 +/- 1.33; rapid eye movement (REM) sleep, 24.4 +/- 0.94; and wake, 39.4 +/- 1.17. Under laboratory conditions, ferrets spend over 60% of the time in sleep and 40.28% +/- 0.93% of total sleep time in REM sleep. The high amount of REM sleep is achieved by having a high number of REM sleep episodes rather than long REM periods. Sleep cycle length was computed in two ways: with long wake episodes removed, 16.7 +/- 0.4 minutes; and with all wake removed, 13.2 +/- 0.3 minutes. Sleep and waking indices in the ferret are compared to those in the cat and discussed with respect to predictions based on several constitutional variables. The high amount of REM sleep and the relative immaturity of the ferret at birth lends additional support for a functional role of REM sleep on ontogenetic development.