The ontogeny and transplacental inducibility of UDP-glucuronosyl transferase (UDPGT) activities potentially relevant to detoxification of polycyclic aromatic hydrocarbons were studied in (C57BL/6 x DBA/2) F1 or (DBA/2 x C57BL/6) F1 fetal mouse liver, with p-nitrophenol (PNP) and 3-hydroxybenzo[a]pyrene (3-OH-BP) as substrates. Both UDPGT activities developed during the late fetal period and reached almost 60% of the adult activity at term; PNP, but not 3-OH-BP UDPGT decreased significantly on postnatal day 1 before rising to adult levels. A single exposure to beta-naphthoflavone (beta NF; 150 mg/kg) on day 17 of gestation induced the PNP-UDPGT activity significantly (1.5-fold) by day 19 in the B6D2 F1s but not D2B6 F1s. A single dose of 3-methylcholanthrene (MC; 100 mg/kg) or 2,3,7,8,-tetrachlorodibenzo-p-dioxin (10 micrograms/kg) did not induce, but three injections of MC also resulted in significant induction in the fetuses of C57BL/6 mothers. 3-OH-BP-UDPGT was not significantly induced by any of the chemicals in either genetic cross. In a parallel study, a gene for an inducible mouse UDPGT, designated UDPGTm-1, was shown by Northern blotting to be expressed in fetal liver by day 18 of gestation at low levels relative to adults, but was not induced transplacentally by MC, beta NF or phenobarbital (PB). These results show that (1) at least two functionally defined UDPGT activities toward phenolic substrates are present in the late fetal mouse liver; (2) one of these is transplacentally inducible by beta NF and MC, but only in fetuses of C57BL/6 mothers, (3) induction where achieved was relatively small in magnitude, and (4) a gene of a PB-inducible UDPGT was expressed at low levels in the fetuses but was not induced transplacentally.