Fourteen piglets, initially weighing 8 kg, were each fitted with a Post Valve T-Caecum (PVTC) cannula and two catheters, one in a jugular vein and one in a carotid artery. Piglets received one of four experimental diets containing either skimmilk powder (SMP), soyabean meal (SBM), soya protein isolate (SI) or fish meal (FM) as the sole protein source; the crude protein content in the diets was from about 17 to 18.4% of DM. The diets were given in equal amounts at 12 h intervals. ¹⁵N-L-leucine was intravenously infused continuously at a rate of about 40 mg ¹⁵N-L-leucine per kg body weight per day. Apparent and true faecal nitrogen (N) digestibilities were 94.0 and 97.3, 84.0 and 93.5, 90.8, and 98.1 and 89.1 and 96.4% for the SMP, the SBM, the SI and the F M diets, respectively. Daily endogenous faecal N losses were 315, 963, 715 and 697 mg and daily endogenous ileal N losses were 786, 1422, 1970 and 1558 mg for diets SMP, SBM, SI and FM, respectively. Apparent ileal nitrogen (N) digestibilities were 84.4, 76.5, 78.4 and 73.0%, true ileal N digestibilities were 92.7, 90.6, 98.4 and 89.3% for diets SMP, SBM, SI and FM, respectively. It was concluded that true N digestibilities were high for all four protein sources, both at the ileal and at the faecal level. Differences in apparent N digestibility were mainly caused by differences in endogenous N secretion.