Oscillations in plasma cortisol levels of newborns during exchange transfusion.

Plasma cortisol concentration was determined in blood samples obtained in six newborns during exchange transfusion at 3--5 minute intervals. A radiotransin-assay using horse serum as a binding protein and toluene-based scintillation fluid for separation of unbound tracer proved to be a relatively specific method for determination of cortisol level in the plasma of newborns. Irregular oscillations were observed in four newborns, whereas a single peak could be demonstrated in two cases during the 40--70 minute period observed. The results suggest that the CRF-ACTH-cortisol secretion may already be episodic in the newborn period.