Fig. 3. Postprandial concentrations (nmol ml^-1; mean ± s.e.m.) of ammonia and urea in blood plasma collected synchronically from the hepatic portal vein (HVP) and dorsal aorta (DA) in free-swimming rainbow trout after a single meal [ammonia in HPV (black squares), ammonia in DA (white squares), urea in HVP (black triangles) and urea in DA (white triangles)]. ^*Statistically significant difference (P<0.05, ^**P<0.01, ^***P<0.001) between the DA and HPV values at a given sample time for either ammonia or urea. Values from the same vessel followed by the same letter do not differ significantly (P>0.05).