1. Wether lambs of 29–44 kg live-weight, totally nourished by the infusion of volatile fatty acids (VFA) into the rumen and casein into the abomasum, were given five treatments in consecutive periods. The treatments were (daily amounts per kg live weight (W)0.75): (a) high-protein for 7 d (2500 mg nitrogen, 650 kJ VFA); (b) low-protein for 7–15 d (525 mg N, 650 kJ VFA); (c) N-free for 7 d (no N, 450 kJ VFA); (d) very-low-protein for 24–28 d (300 mg N, 400 kJ VFA); (e) high-protein for 40 d (2500 mg N, 650 kJ VFA). Nine lambs were subjected to treatments (a), (b) and (c) (Expt 1) and four of the lambs additionally received treatments (d) and (e) (Expt 2).
2. In Expt 1 all nine lambs had a positive N retention on treatment (a) but abrupt change to treatment (b) resulted in substantial negative N balances initially, and a period of approximately 5 d adaptation was required before N equilibrium was re-established. Animals again exhibited negative N balances when the N-free infusion (treatment c) was introduced and during that period there was no evidence of adaptation. Basal urinary N excretion was estimated to be 356 (SE 12) mg N/kg W0.75.
3. In Expt 2 all four lambs were depleted of N when receiving the very-low-protein treatment (d). The progressively decreasing N losses recorded during days 1 to 12 of the treatment period were slightly greater than those recorded during days 13 to 28 but the difference between the means was not significant (P > 0.05). There was no evidence of an adaptation in N retention between days 13 and 28 of the treatment. As assessed during days 13 to 28 of the treatment the efficiency of utilization of infused casein N was 1.0; this compared with a value of 0.66 recorded during treatment (b) in Expt 1. Live weight loss during the period of N depletion was 101 (SE 27) g/d.
4. When lambs were given treatment (e) during the last period of Expt 2, N repletion was rapid and complete within a few days. Ten days after the introduction of the treatment the rate of N retention was estimated to be 1019 (SE 38) mg/kg W0.75 per d and this value declined at a rate of 9.5 (SE 1.9) mg N/kg W0.76 per d for the following 30 d. In comparison, N retention determined for the high-protein treatment in Expt 1 was 724 (SE 66) mg N/kg W0.75 per d. Live-weight gains during N repletion were 292 (SE 26) g/d.
5. It is concluded that N-depleted lambs can replete rapidly and that enhanced N accretion (compensatory growth) may persist for 4–5 weeks. If the improved efficiency of utilization of infused N observed during N depletion reflects a changed basal N requirement, the validity of simple factorial systems for estimating N requirement is called into question.