Background & aims: An oral [ 13 C]urea protocol may provide a simple method for measurement of urea production. The validity of single pool calculations in relation to a reduced sampling protocol was assessed.Methods: In eight fed and five fasted piglets, plasma urea enrichments from a 10h sampling protocol were measured following an intragastric [ 13 C]urea bolus. Blood [ 13 C]bicarbonate was measured to trace gut [ 13 C]urea oxidation. Two-compartment and regression (single pool) computations were performed. Pool sizes were compared to urea distribution over total body water (TBW). Shorter protocol duration was tested in regression simulations.Results: Differences in urea kinetics between fed and fasted piglets did not reach statistical significance. Mean (±SE) urea pool from TBW times plasma urea concentration was 2.2±0.16mmolkg −1 . Two-compartment modelling yielded similar results for pool size (despite the oxidation of a small amount of urea tracer). Urea appearance rate was 306±18μmolkg −1 h −1 . Regression calculations overestimated urea appearance rate vs. compartmental model (P<0.05). When samples <2h were discarded, results were comparable to compartmental calculations even if protocol length was 6h (325±24μmolkg −1 h −1 , NS).Conclusions: Regression calculations using plasma enrichments sampled between 2 and 6h after oral [ 13 C]urea administration provide accurate rates of urea production, and are not affected by tracer oxidation.