With 9 figures in the text) Increase in body mass is an overall measure of growth which hides the dynamics of changes in body components. A study of the allocation of resources to growth versus reproduction is particularly interesting in guinea pigs (Cuviuporcellus) where females mature early in the growth phase and produce highly precocial young while still growing rapidly.In order to get information on energy allocation in guinea pigs, fat-free mass (FFM) of young (1-30 days) and adult females (reproducing vs. non-reproducing) was determined by measuring Total Body Electrical Conductivity (TOBEC; indexed by TOBEC#) which was calibrated by Soxhlet extraction.FFM was predictable from body length alone, but was best predicted as a function of (TOBEC#*length)' '. Prediction was further improved by separating the data into two sets, for animals of < 23 cm and of > 23 cm body length. Fat and total body water (TBW) can also be determined from TOBEC#, but the reliability of fat determinations is low unless samples of 5-10 equally treated animals can be averaged. The formula developed by for the estimation of TBW has frequently been used for all kinds of mammals, but is shown consistently to underestimate TBW even in guinea pigs. Using our calibrations, we studied the development of FFM and body fat in young guinea pigs from birth to 30 days, and the development of FFM and fat stores in reproducing, primiparous, and non-reproducing females of equal age.Young were born with about 11% fat. Young in large litters (3-5) had relatively and absolutely less fat reserves than young in small litters. In eight young measured on days 1, 3, and 5, fat declined until day 3 and returned to the mass at birth on day 5. Fat mass increased slowly during the lactation period, until day 20, and faster thereafter in weaned young. Relative to body mass, fat decreased from 11.2% at birth to 9.3% on day 30, with a minimum of 6.6% on day 20. Mass of fat on day 30 was independent of litter size. FFM began to grow linearly from day 5 onwards. Body water in FFM varied with age in an inverted U-shaped pattern.Reproducing females grew in FFM at equal rates to non-reproducing females, but deposited less than half as much fat until day 210, by which time they had had two litters. Under ad libitum food conditions and a mild thermal regime, reproducing females allocated as much of the available resources to growth in FFM as non-reproducing females, but reduced storage of fat, instead producing biomass of precocial young.