Plasma concentrations of oxytocin (OT) were determined by a highly specific and sensitive radioimmunoassay in (1) nine pregnant women near or at term who were given 400 units of buccal OT every 20 minutes to induce labor or to augment uterine contractions; (2) four adult males who were given 200 units and 400 units of buccal OT every 20 minutes in two separate experiments each lasting 2 hours; and (3) three adult males at regular intervals up to 45 minutes after discontinuation of buccal OT. Plasma concentrations of OT increased in all the women studied, and exceeded 50 picograms per milliliter in six of nine patients after buccal OT was given. However, the net increase in OT was less than 50 pg/ml in six of nine patients. In males, 90% of the plasma samples collected when 400 units of OT was given had detectable levels of OT, with mean levels of 24 to 32 pg/ml; when 200 units was used, only 53% of the plasma samples had detectable OT, and mean levels were consistently below 10 pg/ml. Plasma OT decreased to one third but not to baseline levels in the course of 45 minutes after OT was discontinued. The findings indicate that with 400 units of buccal OT every 20 minutes, plasma OT concentrations attained were similar to those found during the first stage of labor, and that the disappearance of OT from the plasma after discontinuation was slow.