Work at The Hannah Research Institute has demonstrated the effects of an inhibitory (autocrine) protein, within the whey fraction of milk, on the efficiency of milk secretion and responses to changes in milking frequency. The regulatory mechanism involves both the concentration of the inhibitor and the site of milk storage (relative to its site of action, the secretory tissue). Milk storage characteristics influence responses to a number of management practices, most notably changes in milking frequency (Dewhurst and Knight, 1992; Knight and Dewhurst, 1992). There is, therefore, a need to understand phenotypic sources of variation in the sites of milk storage in order to define standard conditions and procedures for making cistern measurements.