The objective of the present study was to measure plasma levels of cholecystokinin (CCK-8 and CCK-33,39) as well as of gastrin during the menstrual cycle and pregnancy. Cholecystokinin and gastrin levels were measured by radioimmunoassay. Before being assayed for cholecystokinin, plasma samples were submitted to HPLC which allowed separation of gastrin and cholecystokinin as well as between CCK-8 and CCK-33,39. Fasting CCK levels were 5.2 ± 0.6 and 7.1 ±0.9 pM during the follicular and luteal phases of the menstrual cycle, respectively. the difference was significant (P<0.05). CCK levels were 8.7±1.2, 10.1 ± 1.6 and 10.4 ± 1.2 pM during the first, second and third trimester, respectively. CCK levels during pregnancy were significantly higher than during the menstrual cycle. the ratio between CCK-33,39 and CCK-8 appeared to increase during pregnancy. Gastrin levels remained unchanged during the menstrual cycle and pregnancy. the role of the high levels of cholecystokinin may be to stimulate the exocrine and endocrine pancreatic function during pregnancy. Furthermore, since cholecystokinin inhibits gastric emptying, it may play a role in the sickness of early pregnancy.