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The amount of energy mobilized or stored as protein, expressed as a proportion of the total energy stored or mobilized (defined as the P ratio, Payne & Dugdale, 1977), was investigated in the young male (rapid growth) and adult female (slow growth) rat. Energy mobilization was induced by a 3 d fast and the changes in body content of fat and lean tissues were used to estimate the fasting P ratio (Pfast). Tissue storage was subsequently effected by 17 d of refeeding and the corresponding ratio (Prefed) was calculated from the amounts of lean and fat tissue regained. The same experimental protocol was used for measuring Pfast and Prefed in weanling (30d) and adult (130d) rats. Weight-matched individuals were assigned to three groups. All animals in group 1 were killed at the beginning of the experiment. Animals in group 2 were fasted for 3d and then killed. Group 3 animals were first fasted for 3d, then had free access to a stock diet for a further 17 d before being killed. During fasting, a close linear relationship was found between weight change and body energy changes (r 0·985, and r 0·92, P < 0·001 for weanlings and adult rats respectively) and between carcass N loss and urinary N loss (r 0·933, P < 0·001). These relationships were used to estimate the body energy and N content of each animal at the end of the initial fasting period. Hence, both Pfast and Prefed values could be calculated for all individuals at both ages. When Pfast and Prefed were calculated for adult rats, the mean values were similar (0·138 (SE 0·002) and 0·130 (SE 0·006) respectively). Individually, the Pfast, and Prefed values were correlated (r 0·54, P = 0·058). In weanling rats, the mean Prefed value was about 2·5 times the Pfast (0·421 (SE 0·0113) v. 0·156 (SE 0·004)). Despite the differences in Pfast and Prefed, the individual values were again correlated (r 0·668, P < 0·05). These results support the suggestion made by Payne & Dugdale (1977), that particular P- ratio values are characteristic of individuals and describe the extent to which protein is mobilized or stored during fasting or refeeding. These observations are discussed in relation to the predictions made by the Payne-Dugdale model. [ABSTRACT FROM PUBLISHER]