A total of 14 healthy pregnant women between 37 and 41 weeks' gestational age were studied to determine the effects of a 5-second external vibratory stimulus (100 Hz, square wave) on human fetal behavioral states. Fetal behavioral state observations were made with simultaneous recordings of fetal heart rate, gross fetal body movements, and fetal eye movement between 8:30 AM and 4:00 PM. During the first episode of behavioral state 1F (quiet sleep state) greater than or equal to 10 minutes duration on two consecutive half days, the vibratory stimulus was placed on the surface of the maternal abdomen over the fetal head. Each fetus received one 5-second vibratory stimulus during either the first or second half day of observation. The time at which the instrument was applied but no vibratory stimulus was administered was considered as the control period. All women were then observed for 2 hours after the control period and after stimulation. A transition period from a quiet sleep state to a stable active sleep state (state 2F, rapid eye movement sleep) averaged less than or equal to 3 minutes after stimulation, as compared with 23 minutes after the control period. These changes in fetal behavioral states persisted for 20 minutes after vibratory stimulation. There also was an immediate and sustained increase in long-term fetal heart rate variability, the number of fetal heart rate accelerations and gross fetal body movement during the first 20 minutes after stimulus, without changes in basal fetal heart rate. We hypothesize that changes in fetal behavior seen after application of low frequency vibratory stimulus are more physiologic than the profound changes in fetal heart rate and gross fetal body movement previously described after vibratory acoustic stimulation with an electronic artificial larynx.