We have shown that serum bile acid concentrations are elevated in human infants reflecting physiologic immaturity of the enterohepatic circulation. To define further the ontogeny of bile acid metabolism in mammals, we examined maturational changes in the serum concentration of total cholate conjugates by radioimmunoassay in fetal, neonatal, suckling, and mature Sprague-Dawley rats. Fetal (21st day) levels were low (1.4 ± 0.23 μM; X̄ ± S.E.), possibly due to minimal enterohepatic cycling in utero. The concentrations in samples obtained minutes after birth, prior to suckling were significantly elevated (12.6 ± 2.37; p < 0.001 vs. fetal); these high values persisted after feeding was initiated (6 hr = 13.5 ± 0.99), but fell at 12 hr (5.2 ± 0.81) and remained unchanged for the duration of the first day. Serum values fluctuated briefly, being 9.6 ± 0.73 on Day 4, and then rose progressively through the suckling period (Day 10 = 7.2 ± 0.57; Day 14 = 10.4 ± 1.70; Day 21 = 16.8 ± 1.93); there was a dramatic peak after weaning (Day 28 = 21.8 ± 1.53). The serum concentration of cholate conjugates then fell (5.6 ± 0.68 on Day 42) to achieve adult levels by 56 days (4.0 ± 0.55), a value substantially different from that of all developing animals (p < 0.025). These data corroborate studies which suggest that a period of physiologic cholestasis occurs in the developing rat. Serum cholate conjugate concentrations likely reflect interrelated morphologic alterations (bile canalicular structure and portal blood flow) and physiologic changes (bile acid synthesis, pool size, and transport) occurring in the enterohepatic circulation during early life.