1. The composition of mammary secretion has been followed in seven women before and after abrupt termination of breast feeding. The period of full lactation was 39 days for 1 woman and a mean of 332 days (range 251--443 days) for the six others. Small samples of mammary secretion (0.5--5.00 ml.) were collected by manual expression from three women at monthly intervals throughout 12 months of lactation and from seven women at frequent intervals for 42 days of involution. 2. During full lactation (12 months, three women) the mean values (+/- S.E. of mean) for lactose, total protein, alpha-lactalbumin, Na, K and Cl were 7.03 +/- 0.13 g/100 ml., 1.68 +/- 0.08 g/100 ml., 163 +/- 6.39 mg/100 ml., 8.5 +/- 0.90 mM, 13.4 +/- 0.34 mM and 11.93 +/- 0.53 mM, respectively. 3. After termination of breast feeding, the concentrations of lactose and K decreased while Na, Cl, fat and total protein increased progressively over 42 days. The increase in the protein content was contributed to by increases in the concentration of lactoferrin, IgA, IgG, IgM, albumin, alpha-lactalbumin and casein. There was no significant difference in the concentration of the milk constituents between the right and left breast throughout full lactation and after its termination. 4. These observations indicated that the secretory capability of the mammary gland of women changed dramatically after complete cessation of breast feeding but that the involuting gland remained partially functional for a long period.