An experiment was carried out to determine endogenous excretion of allantoin (AN) in lambs and kids fasted and fed on a purine-free diet (milk replacer: MR). After weaning (40–50 days after birth), the animals were reared by feeding on MR until they were 120 days old. At 8 months of age, they were fasted for 8 days, and then re-fed on MR for 13 days. After weaning, AN excretion gradually decreased almost to a constant level at 80–120 days after birth (mean 0·14 mmol/kgW0·75/day). In goat kids, there was variation but no definite trend in urinary AN excretion in the period 40–120 days (mean 0·35 mmol/kgW0·75/day). Urinary AN excretion was very low after fasting for 8 days, mean values (per kgW0·75/day) being 0·13 mmoles for lambs and 0·17 mmoles for goat kids. In lambs, urinary AN excretion did not increase following subsequent re-feeding of MR, and for goat kids the increase was small. Changes in plasma AN concentration after fasting and re-feeding showed a similar response to that of urinary AN excretion for both sheep and goats so that there was a high positive correlation between the plasma AN and urinary AN excretion (r=0·881 for lambs and r=0·853 for kids). It is concluded that, in these young animals, urinary AN excretion rapidly responds to changes in plasma AN and is therefore likely to be a useful practical indication of endogenous PD excretion.