To investigate the relationship between beta sympathomimetic drugs and prostaglandins, we measured prostaglandin metabolites in the plasma of pregnant baboons that were given the drug ritodrine. Animals were at a mean gestation of 120 days, which is equivalent to 27 weeks in women. Ritodrine was infused i.v. at a rate of 23 μg/min or 80 μg/min for 4 h. Plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2α (PGFM), 11-deoxy-13,14-dihydro-15-keto-11,16-cyclo-prostaglandin E2 (PGEM-11), and 6-keto-prostaglandin-F1α (6-keto-PGF1α) were determined as indicators of the precursors, prostaglandin F2α, prostaglandin E2, and prostacyclin (PGI2), respectively. At the 23 μg/min ritodrine infusion rate plasma levels of 6-keto-PGF1α were decreased by 51% (± 15%) in the animals receiving the drug (p < 0.005) whereas corresponding levels in control animals did not differ significantly from pre-infusion levels. At the 80 μg/min ritodrine infusion rate, the plasma levels of 6-keto-PGF1α decreased by 66% (± 18%) compared with pre-infusion levels (p < 0.001) but again the levels in control animals were not changed significantly. Plasma levels of PGFM and PGEM-11 in animals during ritodrine treatment did not differ significantly from baseline values. © 1983 J. F. Bergmann Verlag.