Milk samples were obtained from lactating albino guinea pigs after 4 to 6 h of separation from their offspring. The samples were collected in 5-ml glass vials and capped immediately. They were transported rapidly to a pH-gas analyzer for measurements of pH, bicarbonate, base excess, and partial pressure of CO2. Eight guinea pigs were sampled daily for 20 d for a total of 160 observations. Analysis of variance indicated animal and day differences. Regression analyses over days revealed equations of best fit to be quadratic, although stepwise trends were observed in the data as well. The quadratic model for pH was Y(pH) = 6.996 - .057X + .002X2, where X was day of lactation (R2 = .54). The model for bicarbonate was Y (-HCO3 in milliequivalents per liter) = 8.896 - .659X + .0202X2 (R2 = .60). For base excess, the model was Y (in milliequivalents per liter) = -22.65 - 1.99X + .069X2 (R2 = .58). The model for partial pressure of CO2 was Y (millimeters of Hg) = 38.63 + 1.06X - .080X2 (R2 = .33). Decreases in pH and bicarbonate were parallel to changes in mammary epithelial cells of guinea pigs in which degeneration of mechanisms responsible for two important ingredients relating to milk volume occur. These have been identified previously as reductions in lactose and potassium. Synthesis of the enzyme carbonic anhydrase with its resultant bicarbonate production may be related in some way to these components of milk secretion.