Two acute changes in breast milk composition occur during the ovulatory menstrual cycle of women. To determine whether these changes were peculiar to the mammary gland, the composition of unstimulated whole saliva was followed during the menstrual cycle of women. This investigation showed acute, 3-9-fold increases in the concentration of glucose in saliva during the ovulatory menstrual cycle of lactating women. These changes were synchronous with changes in the concentrations of Na and K in breast milk. In contrast to salivary glucose, milk glucose decreased at these times. Milk glucose values were 1.3-fold lower on these days than on the days preceding and following the decrease. Similar acute increases were observed in salivary glucose of non-lactating women. The total duration of the changes in milk and saliva composition was 12-32 h. No cyclic changes in the concentration of glucose were observed in the saliva from women taking oral contraceptives. Since two different epithelial glands are affected simultaneously during the menstrual cycle, it is likely that a hormonal mechanism is involved. Moreover, since the acute increases in salivary glucose also occurred in non-lactating women, we suggest that the factors causing the changes in milk composition are not just a consequence of lactation and are probably closely associated with the hormones controlling the menstrual cycle in all women.