The N-demethylation of ethylmorphine was studied in liver microsomes from human fetuses and adult patients as well as from human fetal adrenals and kidneys. Unexpectedly the reaction was catalysed at the same rate in fetal (42.3-1277.4 pmol/mg/min in 11 individuals) and adult microsomes (414-1617.8 pmol/mg/min in two individuals), which also had similar values of the apparent Km (1.50, 1.72 mM respectively) and Vmax (1.33, 1.81 nmol/mg/min respectively) in studies of the enzyme kinetics. There was a close correlation (r = 0.96) between the semiquantitative immunoblotting assessment of cytochrome P-450 HL-p in fetal liver microsomes (with the use of a monoclonal antibody against pregnenolone-16-alpha-carbonitrile induced rat hepatic cytochrome P-450) and the catalytic activity. The fetal adrenal microsomal N-demethylation was only 11-30% of the hepatic activity when compared within three fetuses in which such a comparison was possible. No activity was measurable in the kidneys. Two drugs that are believed to be substrates of the cytochrome P-450 HLp were tested as inhibitors of the ethylmorphine N-demethylation in human fetal and adult liver microsomes and in rat liver microsomes. Midazolam was a potent inhibitor (100% at 0.4 mM) of the reaction in all specimens, whereas cyclosporin A inhibited the reaction clearly only in adult liver microsomes. Endogenous steroids of importance in the fetal circulation were also tested as inhibitors. Progesterone and dehydroepiandrosterone inhibited the reaction by 75-80% at a concentration of 0.4 mM, whereas pregnenolone and 17-alpha-hydroxyprogesterone were almost devoid of inhibitory potency. These results are of interest in the discussion about the physiological role of the human fetal cytochrome P-450 HLp which has an unprecedented relative abundance in the liver.