Chronic exposure to anesthetic concentrations of halothane during the prenatal and early postnatal periods inhibits the incorporation of the leucine into myelin subcellular fractions in the rat. The enzyme 2',3' - cyclic nucleotide 3' - phosphodiesterase (CNPase) has been widely used as a myelin marker. To determine the effect of halothane on the developmental profile of CNPase, two groups of pregnant Sprague Dawley rats were exposed to 500 p.p.m. or 250 p.p.m. halothane, eight hours per day, five days per week from the third day after conception through postnatal day ten. Control animals were exposed to air alone. CNPase activity was significantly decreased by 500 p.p.m. halothane (34%) and by 250 p.p.m. halothane (29%) at postnatal day 17. Brain and body weights in both halothane treated groups were also less than control animals throughout the measurement period. The data indicates that chronic pre- and postnatal halothane exposure at low levels delays myelination in the rat.