[ 15N]-alanine and [ 15N]–valine dynamics were studied in 29 -30 days pregnant New-Zealand rabbits. Over the experimental period, there was no detectable significant difference of mean ± SD of alanine concentrations within the sampling intervals in maternal, umbilical venous and arterial blood samples suggesting that alanine concentration was in a steady state and that the system was not being perturbed. Similar results were obtained for valine. The enrichment of alanine in the umbilical venous blood was 68.9 ± 2.4% lower than that simultaneously obtained from the maternal vein (P < 0.05). A decrease of 42.7 ± 4.7% in the umbilical arterial sample was observed compared to umbilical venous blood. The data suggest possible fetus production of alanine in utero. Both the maternal and foetal compartment were in isotopic steady state. Comparison of the enrichment of valine in the umbilical venous blood was 54.8 ± 6.3% less than that of the maternal blood. Statistical comparison of the umbilical venous and arterial showed no significant difference. This suggests that there was no apparent valine production during the experimental period