Objective: To investigate whether intramembranous absorption occurs in the rhesus monkey and its role in amniotic fluid (AF) volume regulation as a possible model for the human fetus. Materials and Methods: We studied five chronically catheterized rhesus monkey fetuses (Macaca mulatta) at 126 ± 1 (standard error) days' gestation (term approximately 165 days) with ligated esophagi and catheterized tracheae. Samples (0.5 mL each) of fetal and maternal blood and amniotic and lung fluid were collected at 0, 15, 30, 60, 120, 180, and 240 minutes after injection of 0.1 mCi (3 mL) of technetium-99m (Tc-99m) into the amniotic cavity. Results: In spite of esophageal ligation, there was a rapid absorption of the Tc-99m into the fetal circulation within 15 minutes of injection. The maternal Tc-99m activity increased in parallel to fetal activity but remained lower. The fetal lung fluid Tc-99m activity increased more slowly but was equivalent to the fetal circulating level by 4 hours. Conclusions: These results suggest that intramembranous absorption occurs and may play an important role in rhesus AF volume regulation and composition. Furthermore, this animal model, which closely resembles the human, may provide valuable insight into abnormalities of human AF volume regulation.