Chronic intrauterine monitoring of electrocortical activity, eye movements, and electromyographic activity of the nuchal muscles has permitted the use of established polygraphic criteria to study the ontogenetic development of organized sleep-wake patterns in the fetal lamb in utero. Sixty-five polygraphic recordings obtained from 21 fetal lambs ranging from 112 to 144 days of gestation (term = 145 days) were analyzed. The results show that the transition from disorganized to organized behavioral state patterns occurs at approximately 115-120 days (0.8 term). The physiologic characteristics of the arousal, quiet sleep (QS), and REM sleep (REMS) states are qualitatively similar to those in the adult. There is initially a high proportion of REMS (greater than 50%), with only 11% time spent in arousal. The direct transition from arousal to REMS without a QS period is frequently seen in the fetus at all ages. Quantitative changes in the distribution of behavioral states continue throughout gestation. There is a significant decrease in the incidence of REMS, with a small increase in arousal and QS. The duration of QS and REMS episodes increases with age until a steady level is achieved by 130 days. There is also an increase in the frequency of occurrence of arousal periods, with no change in episode duration.