The activity of the microsomal glucuronyltransferase (GT) was measured in 34 fetal and 27 adult human livers with 2-naphthol as substrate. The average (+/- SD) enzyme activity was 0.07 +/- 0.07 nmol/min/mg protein (fetal) and 7.98 +/- 4.19 nmol/min/mg protein (adult) livers. The adult to fetal ratio of the GT activity was 114. The activity of the cytosolic sulphotransferase (ST) was measured with 2-naphthol as substrate in 30 fetal and 23 adult livers. ST activity (mean +/- SD) was 0.18 +/- 0.12 nmol/min/mg protein (fetal) and 0.63 +/- 0.22 nmol/min/mg protein (adult). The adult to fetal ratio of the ST activity was 3.5. The postnatal development of GT is more marked than that of ST. In the fetal livers, the rate of 2-naphthol sulfation correlated (p less than 0.01) with the rate of 2-naphthol glucuronidation, whereas they did not correlate in the adult livers. No relationship was observed between the activity of the GT or ST and gestational age. The correlation existing between ST and GT in human fetus might suggest that the two enzymes are under a common developmental pattern which seems to be independent of gestational age. The activity of the GT with morphine as substrate was measured in 25 fetal and 29 adult livers and found to be 0.23 +/- 0.20 nmol/min/mg protein (fetal) and 1.85 +/- 0.98 nmol/min/mg protein (adult). Thus, the adult to fetal ratio of morphine glucuronidation was 8.(ABSTRACT TRUNCATED AT 250 WORDS)