Behaviour can be described in terms of its changing uncertainty or decision structure over time. Such a description is economical, maximally informative and may well be of importance neurophysiologically. We try to show that the methods which are normally used to detect temporal patterning between already recognized behavioural acts can also be used on a finer time scale to detect moment to moment patterns of posture within those acts. From such analyses, it is possible to calculate the 'decisioniness' or 'uncertainty' of different parts of the behavioural sequence. We illustrate this by an attempt to describe the decision structure of the drinking behaviour of domestic chicks, using frame by frame videotape analysis. For example, it appears that the first downstroke phase of each drink is more uncertain as to outcome than the other phases, suggesting that 'decisions' are taken during the downstroke. We end with an attempt to plot a continuous graph of behaviour uncertainty against time sampled at 50 msec intervals.