There is currently tremendous interest in how the physiology of individual animals changes and develops during ontogeny. One of the key areas is the extent to which the timing and/or rate of physiological development is fixed within an individual and to what extent can it be altered. We propose that plasticity in the timing of the onset of a particular physiological regulatory system during an individuals development be referred to as physiological heterokairy (to clearly distinguish this phenomenon from physiological heterochrony, which is an evolutionary pattern), and we marshal evidence for three different patterns of heterokairy: 1. altering relative position in the physiological itinerary; 2. altering overall rate of development per se and; 3. a combination of 1 and 2. Using these patterns as a starting point, we develop a framework for investigating physiological heterokairy which takes cognizance of the facts that multiple components of each regulatory system could appear at different times and multiple regulatory systems could come 'on-line' at different times. We finish by placing physiological heterokairy in the wider context of its ecological and evolutionary implications and its relationship to physiological genomics and heterochrony.