Uptakes of oxygen, glucose and lactate by the gravid uterus, fetus and uteroplacental tissues were measured in chronically catheterized pregnant ponies and their fetuses at mid- and late gestation (term 335 days). Rates of O2 uptake by the gravid uterus, fetus and uteroplacental tissues were significant at both gestational ages and were 2- to 3-fold higher in late gestation than the mid-gestation values of 3338 ± 794, 1352 ± 258 and 2035 ± 602 µmol min-1, respectively (n = 4). Similarly, there were significant uptakes of glucose by the gravid uterus, fetus and uteroplacental tissues at both mid- and late gestation. However, unlike O2 uptake, glucose uptake by the uterus and uteroplacental tissues did not increase between mid- and late gestation. No significant uptakes or outputs of lactate were observed by the uterus or uteroplacental tissues at either gestational age, although there was a significant umbilical uptake of lactate in late but not mid-gestation. There was no change in the distribution of uterine O2 uptake between the fetus and uteroplacental tissues with increasing gestational age. The uteroplacental tissues accounted for about 50 % of the uterine O2 uptake at both gestational ages. In contrast, the proportion of the uterine glucose uptake used by the uteroplacental tissues decreased from 73.2 ± 2.1 % (n = 5) at mid-gestation to 61.1 ± 1.9 % (n = 4, P < 0.02) in late gestation. The gestational changes in uteroplacental carbohydrate metabolism in the mare differ from those seen in the ewe and may have important consequences for the duration and outcome of pregnancy in the mare. Experimental Physiology (2000) 85.5, 539-545.