Amniotic fluid clearance of 133xenon was measured in pregnant baboons near term during a control period of spontaneous uterine activity and during induction of contractions with oxytocin or PGF2 alpha and inhibition of activity by a beta-adrenergic agent, berotec. There was no significant change in the clearance of 133xenon from the amniotic fluid during moderate uterine activity induced by oxytocin or PGF2 alpha. The clearance rate decreased five-fold during infusion of berotec, compared with the control infusion periods, simultaneous with a decrease in uterine activity. When the berotec infusion was discontinued, the clearance of 133xenon immediately returned to the previous level, while the return of uterine activity was more gradual.