The aryl hydrocarbon hydroxylase (AHH) and epoxide hydrolase (EH) activities with styrene oxide and benzo[a]pyrene-4,5-oxide as substrates were investigated and compared in the nuclear and microsomal fractions isolated from the human fetal liver, adrenals, kidneys and lungs. The purity of the fractions was estimated by electron microscopy and found to be around 85% for the nuclear and 90% for the microsomal fractions. All tissues catalyzed the hydration of the two epoxides at significant rates. The EH followed Michaelis-Menten kinetics in all fractions. The highest activities were seen in the liver and the adrenals. The nuclear/microsomal ratios of the EH activity was tissue dependent, being highest in the kidneys and lungs. AHH was measurable in the microsomes of all investigated tissues. As to the nuclear fraction it was detectable only in the adrenals and the liver. The nuclear/microsomal ratio of AHH was four times higher in the adrenals than in the liver. It is concluded that not only the microsomal but also the nuclear fraction of several human fetal tissues have the potential of catalyzing formation and elimination of epoxides.