Voluntary automatic milking is proposed as a system whereby dairy cows can choose to be milked when they want. The aim is to develop a system that can milk and perform all the associated tasks without requiring routine human intervention. (Details of the design and operation can be found in Street et al., 1992.) The system may not be viable unless the cows visit at an appropriate frequency, high enough to generate the 0·10 to 0·15 proportional increase of milk yield from cows milked three or more times per day (e.g. Knight and Wilde, 1993) but not so high as to result in some cows over-using the system. Understanding why cows may want to visit the system is therefore important. The two most important reasons why a cow may attend are likely to be motivation to be milked and motivation to eat. Motivation to be milked may change as lactation progresses. Late lactation cows have been shown to enter a milking parlour later than high yielding cows in one experiment (Rathore, 1982) but not in another (Winter, 1993). Rathore (1982) suggested that motivation to be milked may be generated by the discomfort of a large and distended udder. Motivation to be milked may also be linked to some inherent desire of the cow to suckle and subsequently wean her calf. This may be independent of the amount of milk in her udder and decline during lactation. Therefore motivation to be milked could be generated either from the discomfort of a large and distended udder or by some psychological desire by the cow to suckle a calf generalized to a milking machine. Either way, the strength of motivation to be milked may have implications for how an automatic milking system (AMS) is designed. For example, if cows were highly motivated to be milked and attended at an appropriate frequency, there would be no requirement to provide additional incentives, such as food, to attract cows into the AMS. In addition, if cows choose to be milked more frequently than they are milked in conventional systems (generally twice per day), then this may be a method by which choices can be engineered into an animal’s environment.