The effects of a high- (HF) and a low- (LF) fibre diet on porto-arterial plasma concentration differences and plasma levels of urea ammonia-nitrogen were compared in six Swedish Landrace X Yorkshire pigs (30-52 kg). Portal and arterial blood samples were drawn during 8 h following two consecutive meals, given at 08.00 and 16.00 hours. The HF diet, in comparison with the LF diet, was found to produce significantly lower portal plasma urea levels. However, concomitant arterial plasma were equally depressed, consequently leaving the porto-arterial urea differences unchanged. In no instance during the 16 h studied could negative porto-arterial urea differences clearly be observed. It was thus concluded that none of the diets induced an important net flux of plasma urea directly from the circulation into the gastrointestinal tract. The significantly lower circulating plasma urea levels that were observed when the pigs received the HF diet were also associated with significantly lower urinary excretions of urea. The HF and LF diets had similar effects on portal and arterial plasma levels of ammonia-N. This was also true with regard to porto-arterial ammonia-N differences.