The percentage of rapid eye movement (REM) during sleep is substantially greater in neonates (infants in first month after birth) than in other children or adults. It was hypothesized that REM rate may decline as rates of many response sequences do when repeatedly elicited. Electrical recordings of eye movements were obtained from a 3-day-old male and followed through his seventh day, from 25 females aged 3, 4, or 5 days, and a kitten observed 8 hours after birth and for the next 5 days. The REM rate was obtained under noise conditions and quiet conditions. The critical statistic was the eyes-closed REM rate, and results supported the hypothesis. The rate for 5-day-old infants was significantly lower than for 3-day-old infants. Differences over short time periods and records of individual REM rates for infants and the kitten were in the expected direction. Neurological and psychological significance can be attributed to the developmental change in REM sleep, of which sleeping REM rate is a particularly promising indicator. (DR)