The effects of exogenous purine supply on plasma concentration and urinary excretion of purine derivatives (PD), which include allantoin, uric acid, xanthine and hypoxanthine, were studied. Five sheep, totally nourished by intragastric infusion of volatile fatty acids and casein, were given an abomasal infusion of a mixture of adenosine and guanosine at three levels (5·0, 10·0 and 20·0 mmol/day) each in four infusion patterns (as one, two, or four 3-h infusion periods per day or infused continuously, P1 to P4 respectively). Urine was collected hourly over 24 h, and plasma samples were collected hourly for 7 h from the start of purine infusion. Both the plasma concentration of PD and its urinary excretion changed rapidly in response to the start and termination of purine infusion with a lag time of 2–3 h. Individual sheep differed considerably in the relative proportions of the different purine derivatives in the urine. The endogenous urinary PD excretion averaged 176 (±S.E. 28) μmol/kg W^0·75 per day. Daily PD excretion in urine increased with the amount of purine infusion, but at each level the output decreased significantly in a gradient from P1 to P4. The response of total PD excretion (Y, mmol/day) to daily purine input (X, mmol/day) was predictable and followed the equation: Y=0·81 (±0·02)X+3·07(±0·81) × (u e^−0·27X/u +1−u), where u is the duration over which the daily input was infused expressed as a proportion of 24 h. The recovery of the infused purines averaged 81%. When the infusion was given continuously, plasma PD concentration was relatively stable and was linearly correlated with the purine entry rate and daily PD excretion in the urine. The estimated glomerular filtration rate was 150 (±S.E. 16) litres/day and the estimated tubular transport maximum was 2·6 (±S.E. 1·3) mmol/day. It is concluded that due to the rapidity in the response of plasma and urinary PD to changes in the exogenous purine supply, spot measurements of PD in urine or plasma can be of no value for the estimation of exogenous purine uptake unless the purine supply is relatively constant throughout the day. Daily PD excretion in urine related to the exogenous purine uptake in a predictable and reproduceable manner and provides a reliable index for the estimation of the exogenous purine uptake.