The average concentration of orotic acid in the milk of 412 Black and White bred cows from four Polish provinces was 0.618 +/- 0.233 mmol/l. There was no correlation between milk yield and concentration of orotic acid. A higher concentration of this pyrimidine in younger cows, and its increase during development of lactation was noted. The yearly pattern of orotic acid in milk and urine of four low-, and four high-orotate cows was examined. In spite of high average differences in milk orotate (0.397 and 0.813 mmol/l) no significant differences in urinary orotate (20.96 and 21.90 mumol/mmol creatinine) were observed. In both groups the lowest milk orotate level occurred in early lactation. The orotic acid content (mmol/l) in commercial milk products was as follows: skim milk--0.783; evaporated milk--0.538; cream 12% fat--0.367; buttermilk--0.449; yogurt--0.331; kefir--0.341; sour milk 2% fat--0.360; dried skim milk--1.042; Bebiko I (infant formula)--0.650. Leukemia led to the elevation (0.845 mmol/l), whereas mastitis to the depression (0.124 mmol/l) of milk orotic acid level.