Oral glucose tolerance tests (OGTT) were carried out in 9 normal pregnant women and 11 non-obese gestational diabetics in late pregnancy. All samples were analysed for the content of glucose, insulin and placental lactogen hormone (HPL). Furthermore, spontaneous changes in the serum HPL concentration during a 3 h period were studied in 6 normal women in the 2nd half of pregnancy. During OGTT only small and insignificant changes in the level of HPL were observed in both the normal subjects and the gestational diabetics. Furthermore, the mean HPL concentration curves of the normal subjects and the gestational diabetics were superimposed although the mean glucose concentration curves were significantly different. The study of spontaneous changes in HPL revealed only small and insignificant fluctuations in the serum HPL level, and the mean concentration curve resembled those obtained from the OGTT-study. It is concluded that neither the absolute serum glucose level, nor physiological fluctuations in the serum glucose concentration seemed to influence the serum concentration of HPL in this type of patients.