Holstein-Friesian male calves, aged 160 to 450 days, and of live weight 180 to 530 kg, were used to determine the effects of dietary nitrogen level and dietary nitrogen source on performance. Experiment 1, tested dietary nitrogen level, and involved two trials with three dietary-nitrogen levels and equal dietary metabolizable energy (ME) 11·7 MJ/kg dry matter (DM). The crude protein (CP) level was reduced in the course of the trials by 40 g/kg, the experimental average CP in the diets being 146, 126 and 106 g/kg for the high (HP), medium (MP) and the low protein (LP) diets, respectively. The urea space (US) for estimation of body protein deposition and the rumen volume for calculation of empty body weight were measured in trial A of experiment 1. Experiment 2 involved four diets of equal ME concentrations; (11-7 MJ/kg DM), in three of which the CP contents were equal but from different sources: (a) 110 g/kg, all of it true protein (TP), negative control; (b) 130 g/kg, all of it TP; (с) 130 g/kg CP, 20 g/kg of it is poultry litter (PL) protein; (d) 130 g/kg CP, 40 g/kg of it is PL protein. Significantly lower US and ratio of US to live weight were found in the calves on the LP diet, from the age of 265 days than in the calves on the other diets. The ratio of US to live weight significantly decreased with increasing age in all protein level treatments. Until the age of 300 days the rumen volume was significantly higher on the LP diet than on the other diets. Calves on the HP diet had the highest daily gain and carcass gain. The recommendation to reduce the dietary protein as age increased to lower than 120 g/kg caused a reduction in the energy retained from the diet but protein deposition was not impaired. Protein deposition was impaired when the CP was reduced by 20 g/kg below the level recommended by the National Research Council. The inclusion of poultry litter in the diet, with ME concentration being maintained, did not reduce the rate of live-weight gain, and improved food conversion efficiency.