The effect of maternal undernutrition on muscle development of near-term ovine fetuses was studied in 2 experiments. In each experiment, Welsh mountain ewes were randomly assigned to either control (C) or nutrient-restricted (R) groups. In the first experiment, mild maternal undernutrition (85% of daily nutritional requirement (DNR)), and in the second experiment, severe maternal undernutrition (50% of DNR), were imposed between 0 and 70 days of gestation (dg). Controls for both groups were maintained on 100% of their DNR throughout the experiment. In each experiment, ewes were killed at 126 +/- 1 dg by an intravenous injection of pentobarbitone. The semitendinosus muscle was dissected, and sections were stained for alkali-stable ATPase. The total number of primary and secondary fibers and the total number of fibers were estimated. In both experiments, there was a significant decrease in primary, secondary, and total fiber numbers (P < 0.05 for each). We speculate that the decrease in primary fiber number in both experiments is due to some of the large, central, and alkali-ATPase-negative (slow) fibers at near-term being secondary fibers. Therefore, the real primary fibers may not be affected.