The change in the amount of immunoreactive oxytocin in human term placental tissue due to cycloheximide, an inhibitory agent of protein biosynthesis, was observed by a tissue culture method. The immunoreactive oxytocin content per gram tissue was 1.81 ng/g and significantly decreased to 1.17-1.11 ng/g when the concentrations of cycloheximide were at 1-10 micrograms/ml in culture medium. The total quantity of immunoreactive oxytocin including medium content was 1.82 ng/g and significantly increased to 3.06 ng/g in the control group. However, those of the groups in which added cycloheximide was at 1-10 micrograms/ml, were 1.91-1.85 ng/g and showed no remarkable changes after incubation. The data suggest that immunoreactive placental oxytocin can be synthesized in the human placenta itself rather than in other tissues.