The present experiment was carried out to study the endogenous losses of purine and pyrimidine derivatives from pregnant sows. Three pregnant and three non-pregnant Large White × Landrace sows were fed on a purine-free diet composed of starch, glucose, sucrose and vegetable oil, with casein as the protein source. The experiment began, for the six animals, after diagnosis of pregnancy and was divided into six 12 d periods. Urine was collected during the first 3 d of each experimental period by means of a urethral catheter for determination of allantoin, uric acid, xanthine, hypoxanthine and pseudouridine concentrations. In the absence of dietary nucleic acids (N A), allantoin and, as a consequence, excretion of total purine derivatives (PD) decreased significantly to a constant value (128·3 (se 7·07) μmol/kg metabolic live weight (W0·7) per d), an amount assumed to represent endogenous excretion. Excretion of uric acid (38·7 (SE 2·15) μmol/kg W0·75 per d), hypoxanthine (21·0 (SE 2·58) μmol/kg W0·75 per d) and xanthine (11·2 (SE 0·83) μmol/kg W0·75 per d) were not affected by the experimental treatment, although there was a significant decrease in hypoxanthine excretion in pregnant sows (from 25·5 to 5·2 μmol/kg W0·75 per d) compared with non-pregnant sows (from 26·7 to 44·8 μmol/kg W0·75 per d). Creatinine excretion was not affected by pregnancy and was used as an internal urinary marker. Purine excretion, either expressed as μmol/kg W0·75 per d or as the ratio PD:creatinine, was not affected by experimental treatment, although an apparent increase in pseudouridine excretion, a modified unsalvageable catabolite of RNA-pyrimidine, was found in late pregnancy (3·6 ν 52 mol/100 mol creatinine in on-pregnant sows compared with pregnant sows at 102 d collection).