This study was undertaken to determine whether differences in fetal activity could account for the unpredictability of absorption of red blood cells from the peritoneal cavity of fetuses with alloimmune hydrops. The absorption of red cells from the peritoneal cavity of fetal lambs was studied in 7 pregnant ewes. In 3 nonparalyzed fetuses, uptake of donor red cells was complete by 80 h. In 4 fetuses given pancuronium for 4 days to abolish breathing and body movements, the absorption of red cells was markedly diminished at 92 h, but was complete 3–4 days after fetal movement resumed. This study demonstrates that fetal movements are essential in ensuring the normal absorption of red cells from the peritoneal cavity. Variability in fetal movements, particularly fetal breathing movements, may explain the unpredictability of absorption of red cells from the peritoneal cavity in fetuses with hydrops fetalis.