In continuation of previous investigations on hormonal contraception (Seng et al. 1969) and on gestation and the puerperium (Becker et al. 1971) in 10 female volunteers with no apparent endocrinological disturbances various parameters of lipid and carbohydrate metabolism were determined on days 7–9, at least on three consecutive days during ovulation, and on days 23–25 during the corpus luteum phase of the menstrual cycle. The concentration of glucose, pyruvate and lactate as well as glycerol, glycerid-glycerol, free fatty acids, acetoacetate and β-hydroxybutyrate in the blood were analyzed and the data obtained to basal temperature measurements, total oestrogen excretion values and blood progesterone levels. Only during the luteal phase, i. e. at the highest progesterone levels was a statistically significant increase of free fatty acids found in the blood, accompanied by a distinct but not significant rise in free glycerol. None of the other parameters measured showed any statistically significant correlations to the various phases of the menstrual cycle. The importance of the increase in free fatty acids on the sequence of events observed at the corpus luteum phase under hormonal contraception and during late gestation is discussed.