The growth of the fetal guinea pig was studied in 32 fetuses in 12 litters, ranging from 39 days gestation to full term (67 days). The wet weight of the fetus was well approximated by an exponential function of gestational age (GA) in days [fetal weight (g) = 0.993 e(0.068 X GA), r = 0.94]. Dry weight increased more rapidly than wet weight [dry weight (g) = 0.039 e(0.102 X GA); r = 0.97], resulting in an increase in percent dry weight from approximately 10% at 40 days gestation to 30% at term. Fat content increased even more rapidly than dry weight [body fat (g) = 0.00123 e(0.136 X GA), r = 0.97], accounting for 33% of dry weight and 11.7% of wet weight at term. Using bomb calorimetric projections of caloric value of 9.3 kcal X g fat-1 X day-1 and 4.6 kcal X g nonfat dry wt-1 X day-1, we estimate that growth of the fetal guinea pig requires 220 kcal X kg fetal wt-1 X day-1 near term. Carbon and nitrogen contents of the fetus increased at different rates, reflecting the changes in fat and nonfat tissues. Amino acids contributed 80% of total body nitrogen and 41% of total body carbon near term. Cysteine concentrations increased and lysine concentrations decreased with gestational age; the concentrations of the other measured amino acids did not change with gestational age. These studies represent the first systematic study of the chemical growth of the fetus in a nonhuman species.