The present study was conducted to evaluate whether bodyweight and the micronisation of dietary fibre affect the endogenous nitrogen and amino acid losses (ENL and EAAL) in pigs. The effect of the micronising process was tested by providing pigs with 90 g DM·kg–1 BW0.75 of a N-free diet supplemented with isolated pea inner fibres, presented in native or micronised form and with a water-holding capacity of 12 and 4 g water·g–1 DM, respectively. ENL and EAAL were measured on pigs weighing 24, 62 and 105 kg. In all cases, daily ENL increased linearly (P < 0.05) with BW, for the majority of the AA and total N. As BW increased, daily ENL, total EAAL and the majority of EAAL increased linearly independently of micronisation (P < 0.05). When expressed per kg DMI, total EAAL and the majority of each EAA decreased curvilinearly and reached nadir at around 100 kg BW. For ENL expressed per kg DMI, micronisation resulted in a curvilinear decrease with increasing BW, as compared to a linear decrease for pigs fed the native pea fibre diet (non-micronised). Micronisation of pea inner fibres did not decrease ENL or EAAL daily, except for proline. When the losses were expressed as g·k–1g DMI, micronisation did not decrease ENL but decreased (P < 0.05) endogenous losses for a majority of AA as well as for total AA. The results suggest that small pigs excrete more endogenous N per kg DMI than large pigs and that pea fibre micronisation reduces EAAL but not ENL when expressed per kg DMI.