The effects of dietary deficiencies of phosphorus (P) and nitrogen (N) were studied in 28 lambs allotted to seven treatments for 18 weeks. The lambs in four treatments were offered roughage-based diets ad libitum containing high (H) or low (L) levels of P (4·13 or 0·98 g/kg dry matter (DM)) and N (108 or 68 g crude protein/kg DM). The lambs in the remaining three treatments were fed the same quantities of DM of the HNHP diet as the lambs receiving the three deficient diets (HNLP, LNHP and LNLP). A dietary deficiency of P caused a reduced DM intake and liveweight gain, lower yields of N and DM in carcass muscle and reduced DM, N and P contents of bone. A dietary deficiency of N resulted in reductions in DM intake, liveweight gain, DM digestibility, N balance, and in carcass muscle and bone DM and N. The deficiencies of N and P were not additive in lowering food intake but were additive in reducing the mineralization of the bones. The DM intake of the lambs offered their diets ad libitum was closely correlated to the log of the plasma inorganic P and urea N concentrations (R =0·72). Feeding lambs with restricted quantities of diet HNHP resulted in higher liveweight gains and DM and N digestibilities than when any of the deficient diets were offered. Measurement of the N content of the lambs by a neutron capture gamma ray analysis technique showed that the changes were occurring progressively over the experimental period. The P content of the metatarsal bone, measured by a neutron activation technique, showed progressive changes. Although the faecal endogenous losses of P were related to P intake, high urinary losses were observed in those lambs which had their food intake limited by low dietary N alone, or had their food intakes restricted. The total endogenous P losses of the lambs were most closely dependent upon the DM intake and plasma inorganic P together. The efficiency of P absorption was high (> 0·75) in all lambs except for those given the HNHP diet ad libitum. The glucose entry rate was reduced in the lambs offered diets deficient in either N or P. Mechanisms by which the N and P deficiencies reduce DM intake are discussed.