Sir.—In Dr Osaki's recent article regarding the association of maternal oxytocin administration and neonatal unconjugated hyperbilirubinemia (Am J Dis Child 129:1139, 1975), he offered several suggestions to explain this observed relationship. One hypothesis was that oxytocin administration might inhibit hepatic bilirubin glucuronyl transferase activity. We investigated this proposed mechanism using a previously reported1.2 in vitro study in which inhibitory activity of human breast milk was demonstrated.
Serial dilutions of 0.2 ml of a 10 units/ml solution of oxytocin was added to the in vitro assay, and equal volumes of physiologic saline were used for control purposes. The results showed no significant inhibition of bilirubin glucuronyl transferase activity (Table) at all concentrations tested. This included supraphysiologic and physiologic levels for the rat.3.4
Although the results of this in vitro study would suggest that maternal oxytocin administration does not in
terfere with bilirubin conjugation in the newborn, the possibility