The objective of this work is to develop non-destructive techniques for assessing the mammary secretory mass of dairy cows. The nutritional responses of dairy cows depend on characteristics of both the feed and the cow and the latter has received insufficient attention. Experimental animals have generally been described in terms of their liveweight and condition score which gives only a rough indication of the masses of fat and protein. These studies are developing a description of dairy cows which has 3 major components whereby nutritional effects can carry forward within (a) body fat, (b) body protein or (c) mammary secretory mass. Milk yields are closely related to mammary secretory mass across a wide range of species although there is sufficient variation in mammary secretory efficiency (milk yield per unit mammary secretory mass) within a species to encourage selection for increased efficiency. Studies of this kind have been limited by the lack of a non-destructive technique for assessing udder volume; serial slaughter studies of mammary mass are particularly affected by large between-animal variation (e.g. Gibb et al, 1992). Recently, a technique utilising polyurethane foam to prepare casts from oxytocin-milked udders has been developed (Knight and Dewhurst, 1994; Dewhurst et al, 1993) and can be used for repeated measurements within-cow. The technique has been used to investigate changes in mammary secretory mass and efficiency in first-lactation Holstein-Friesians.