In the frame of a feeding experiment with three periods, balance trials were carried out with 18 double-muscled Belgian White-blue bulls, allocated to one of:Four feeding regimes. The ration consisted of maize silage and concentrate in the ratio of 35:65 on DM basis. Six concentrates were formulated to supply three levels of protein and energy. The three periods corresponded to distinct live weight intervals of 360-460, 460-570 and 570-680 kg. In the first feeding regime, low protein and intermediate energy level were given during the whole trial; in the second regime, protein level decreased at a constant intermediate energy level; in the third regime, the energy level increased at a constant high protein level; in the fourth regime, protein level decreased simultaneously with an increase in the energy level. Total daily intake varied from 6.2 to 9.7 kg for dry matter (DM), from 65 to 106 MJ for metabolisable energy (ME) and from 719 to 1326 g for crude protein (CP) (50 animal observations). At the end of each period, the excretion of the purine derivatives (PD), allantoin and uric acid, was measured after total urine collection during 4 days to estimate microbial nitrogen supply to the duodenum (MNPD) The effect of the intake of DM, organic matter (OM), digestible OM (DOM), digestible carbohydrates (DCHO), total digestible nutrients (TDN), rumen fermentable OM (FOM), metabolisable energy (ME), fermentable ME (FME), CP, digestible CP (DCP) and rumen degradable protein (RDP) on PD and MNPD was examined. Further, the relationship of MNPD to MN, calculated according to different systems, was examined. The amount (mean+/-SD) of allantoin excreted in urine was 147+/-23 mmol day(-1) and of uric acid 11+/-3 mmol day(-1). The MNPD amounted to 97+/-21 g day(-1), varying from 57 to 154 g day(-1), and significantly increased with all measures of nutrient intake. The correlation coefficients, ranging from 0.45 for DCP to 0.57 for DOM and FME, were, however, not significantly different. MMPD showed a larger variation and was on average lower than the potential MN values calculated from the intake of FOM (108+/-13 g day(-1)), DCHO (141+/-16 g day(-1)) and FME (109+/-13 g day(-1)), similar to that calculated from the intake of RDP (99+/-14 g day(-1)) and higher than the MN value calculated from the intake of TDN (84+/-11 g day(-1)). The correlations of Mn-PD to the calculated MN values ranged from 0.47 for MNFOM to 0.59 for MNFME, but were not significantly different. (C) 1998 Elsevier Science B.V.