Cyclic AMP exchange among the mother, amniotic fluid, and fetus was studied in normal rhesus monkeys at term pregnancy. Following a pulse intravenous administration of 3H-cyclic AMP into the mother, a small fraction of the dose appeared in less than 1 minute in fetal blood. It appeared in the amniotic fluid after 5 minutes and reached maximum level in 20 minutes. The accumulation of 3H-cyclic AMP in the amniotic fluid in 1 hour was 0.03 per cent of the injected dose. The amount and time course of 3H-cyclic AMP accumulation in the fluid were not altered by tying the umbilical vessels. The transfer of 3H-cyclic AMP injected in utero into the fetal femoral artery resembled that in the mother; in 1 hour the amniotic fluid contained 0.22 per cent of the injected dose. When injected directly into the amniotic sac, more than 65 per cent of the injected dose remained unchanged after 1 hour, with minimal transfer into the maternal and fetal compartments. This study showed rapid bidirectional exchange of cyclic AMP between the mother and the fetus. Both of these compartments can contribute cyclic AMP to the amniotic fluid, independently or in concert. It remained fairly stable in the fluid and was not readily metabolized or transported out.