The ability of the fetal pig intestine to absorb large proteins was investigated in utero. Six pregnant sows were anesthetized (Na pentobarbital) at 99-102 d of gestation (term = 115 ± 2 d), and a catheter was inserted into the esophagus of two to three fetuses per sow. Via these catheters, sterile solutions (10.0 mL) of colostrum whey (CW, n = 5), milk whey (MW, n = 5), or amniotic fluid (AF, n = 4) were infused into the fetal pig stomachs every 6 h for 6-8 d starting on the day after surgery (d 0). Levels of IgG in the three fluids were 120, 0.5, and 0 mg/mL, respectively. During the first 2-3 d of infusion, plasma IgG levels rose rapidly in the CW fetuses (to 7.5 ± 0.8 mg/mL), whereas IgG remained absent in plasma from MW and AF fetuses. Absorption of a macromolecule marker, BSA, was also higher when the marker was given with CW rather than with MW or AF. However, when all three treatment groups were given CW + BSA on the last experimental day (d 6-8), the mean BSA increment in the CW group was only 5-8% of that in the AF group, with intermediate values for the MW group. Neither at the beginning nor at the end of the experiment was macromolecule uptake in individual CW fetuses correlated with their cortisol level in plasma. The prenatal pig intestine is similar to the neonatal pig intestine in that colostrum stimulates both the macromolecule absorption and the cessation of macromolecule uptake (intestinal closure). However, fetal pigs have a lower protein absorptive capacity and a longer preclosure period than newborn pigs; this may be related to an immature structure and function and a slow enterocyte proliferation rate in the prenatal pig intestine.