Different excretion patterns were observed for p-amino- hippuric acid and its acetylated metabolite in mammary excretion experiments in goats and cows. The excretion of p-aminohippuric acid conformed to passive diffusion of the non-protein bound and un-ionized fraction (Rasmussen 1958, 1966) as described by Miller et al. (1967). Thus, the p-aminohippuric acid (pKa 3.8) appeared in ultrafiltrates of milk in concentrations 1/4—1/10 of the concentrations in ultrafiltrates of blood plasma. The concentrations of acetylated p-aminohippuric acid (pKa 3.9) in ultrafiltrates of milk were on the other hand equal to or higher than in ultrafiltrates of blood plasma.