Three pigs of about 33 kg body weight, each fitted with re-entrant cannulas into the duodenum and ileum and with bladder catheters were used for the study. One of the pigs was infused intravenously with ¹⁵N-leucine for 8 days. Total N passage, ¹⁵N nitrogen passage through the duodenum and ileum, and N balance were determined. The mean amount of total N passing the duodenum exceeded the ingested N by 12%, and that passing the ileum accounted for 27% of N intake. Of the infused ¹⁵N, 67.4% was found in the urine, faeces, digesta and carcass after slaughter. Taking the recovered amount of ¹⁵N as 100%, 20.4% was found in urine, 0.6% in faeces, 8.2% in digesta and 70.8% in tissues. It was concluded that ¹⁵N retention from intravenously administrated leucine was much higher than from ¹⁵N in urea given with feed, as estimated previously.