To identify those behavioural patterns that represent ethological needs, the important questions are firstly how strongly motivated are behavioural patterns believed to represent ethological needs, and secondly how do we best provide an outlet for those behaviours. To establish motivational strength one approach is to impose an increasing cost of access to perform the particular behavioural pattern. The performance of the behaviour as a function of the cost imposed may be described by a so-called demand function. Measures derived from this function are the sensitivity to increasing cost (the price elasticity) and the maximum effort the animal will commit for access to perform a bout of the behaviour (the maximum price paid). It has been questioned whether a single price elasticity value may be assigned to the importance of each behavioural pattern, or if a value may only be assigned to a certain quantity of access. The prerequisites for price elasticity to be an indication of motivational strength of a behavioural pattern are outlined, and examples of systematic development of the price elasticity of demand as a measure of motivational strength are given. How do we identify those stimuli that best provide an outlet for the behaviours that represent behavioural needs? In addition to giving the animal a choice it is relevant to impose a cost on the choice. Measures of preference from analysis of cross-points between concurrent demand functions may be used to identify which resources best ensure the satisfaction of the ethological need.