The effect of cocaine on the behavioral state of six fetal sheep was studied during gestational ages between 128-135 days. Two to eight days after surgery, fetuses received either a continuous 60 min intravenous infusion of cocaine HCl (33.4 mg) or saline. The infusions were preceded and followed by control periods of 102 min. Cocaine induced a disruption in fetal behavioral state cyclicity and a decrease in the amount of time spent in rapid eye movement sleep (P < 0.01) and non-rapid eye movement sleep (P < 0.05) during the infusion, but not during the recovery period. Spectral amplitude of the electrocortical activity at all three cortical locations increased within most one-third octave bands between 0.8-4 Hz and decreased within most bands between 16-25 Hz (P < 0.05) compared to controls. There were no differences in spectral amplitude between pre- and post-cocaine periods at any location over the 25 frequency bands studied (P > 0.6) except for one frequency band centered at 12.5 Hz. The effects of a one hour cocaine infusion on fetal cortical electrical activity are diffuse, but short-lived, and occur independently of changes in fetal oxygenation.