There is little information to what extent the secretion of prolactin (HPRL) can be stimulated in pregnancy. The effect of thyrotropin releasing hormone (TRH) on the secretion of HPRL and thyroid stimulating hormone (TSH) in late pregnancy was examined with special emphasis on the amount and time sequence of secretion of both hormones. 21 pregnant women and 20 non-pregnant women were tested by intravenous injection of 200 ug TRH. In the pregnant group the HPRL serum levels increased much more than in the non-pregnant group, however, the increase of HPRL above baseline levels expressed as percentage was lower. In contrast, TSH secretion was similar in both groups. The baseline levels of HPRL did not influence the response to the stimulus. There was no evidence for a correlation between the secretion of HPRL and TSH. The maximal increase of both hormones in serum occurred at various testpoints, however, most of the HPRL peaks preceded those of TSH. Repeating the TRH test after two or three days resulted in good reproducibility concerning HPRL secretion whereas TSH secretion was diminished.