In attempting to explore the mechanisms of interaction of genetic and environmental factors that affect the quantitative requirements of energy by man, the convenience of an animal model was considered and searched for. The idea was to start with male and female Wistar rats and through inbreeding segregate the highly effective users of energy from the poor users. The efficiency of dietary energy utilization was measured by the index of conversion (IC) defined as the dietary intake necessary to increase 1 g of body weight in a 32-day period, from day 21st to day 52nd of extrauterine life. The median value of the IC for all animals included in each experiment was the cut-off point to classify each individual as a good or a poor energy user. The first generation had three males and five females with a median IC = 2.90 and a range from 2.54 to 3.25. The proportion of males below the median was 3/3 while the proportion of females was 2/5. The difference in proportions was striking and led immediately to the consideration of a sex-link hypothesis, and to test it the, IC of 91 Wistar rats randomly selected at birth was obtained. The median value of the series was 2.99 with a range from 2.24 to 3.95. The proportion of male rats with values below the median was 33/38 while the corresponding proportion for females rats was 13/53. In other words, while nine of every ten male animals were good users of energy, only two out of ten females fell into this category.(ABSTRACT TRUNCATED AT 250 WORDS)