Estimates have been made of the irreversible loss of alanine and of glycine in chronically catheterized fetal lambs and in sucking lambs using [U-14C]-labelled radioisotopes. The experiments in the fetal lambs were carried out at least 5 d after implantation of catheters. The mean concentration of glycine in fetal femoral arterial blood between 102 and 129 d conceptual age was 755 mumol/l and this was not significantly different from that in maternal venous blood. The mean concentration of alanine in fetal femoral arterial blood during the same period of gestation was 229 mumol/l and this was significantly greater than that in maternal venous blood. Assuming a catenary model, the mean irreversible loss of glycine, determined using the single-injection technique, in three fetal lambs of 107, 111 and 127 d conceptual age was 17 mumol/min per kg, whereas in two fetal lambs aged 106 and 109 d into which the isotope was infused continuously the mean irreversible loss, calculated from the specific activity of glycine 5 h after the start of infusion of the tracer ('pseudo plateau'), was 12 mumol/min per kg. In a sucking lamb, 9 d after birth, the irreversible loss of glycine was 11 mumol/min per kg. The mean irreversible loss of alanine, determined by the single-injection technique assuming a catenary model in five fetuses between 112 and 121 d conceptual age was 14 mumol/min per kg, and in two sucking lambs, 9 and 11 d after birth, it was 5.1 mumol/min per kg. When a two-pool model was assumed in which entry of metabolite was not directly into the sampling pool but was by way of the second pool, then the mean irreversible loss of glycine in the three fetuses was 23 mumol/min per kg and of alanine in the five fetuses was 32 mumol/min per kg. Calculations based on the alternative two-pool model did not alter appreciably the rates of irreversible loss of either alanine or glycine in the sucking lambs. From a comparison of the specific activities of the amino acids and of carbon dioxide in blood during the course of the experiments, it was found that in the fetuses 0.96% of the CO2 present in blood was derived from alanine and only 0.12% was derived from glycine. It was calculated that not more than 1.6 mumol alanine/min per kg and 0.29 mumol glycine/min per kg could have been converted to CO2 in the fetal lambs.(ABSTRACT TRUNCATED AT 400 WORDS)