1. In the first trial, hourly blood samples were drawn during the light period (06·30–20·30 hours) from thirtyfour dairy heifers in two groups of different ages.
2. Concentrations of serum folates were 8·2 (se 0·1) ng/ml at the age of 18·2 (se 0·6) d and 14·8 (se 0·2) ng/ml at 120·7 (se 2·9) d (P ≤ 0·001).
3. In the second trial, forty heifers were randomly assigned to a factorial experiment where age (2 weeks v. 4 months) and quantity of pteroylmonoglutamic acid injected intramuscularly (2·5, 5·0, 10·0 and 20·0 mg) were the two factors studied. Blood samples were taken immediately before injection and on days 1,2,3,4,7 and 10 after the injection.
4. Serum folates reached a maximum on day 1 after the injection of pteroylmonoglutamic acid. The response to the injection was different for the two age groups (P ≤ 0·0002); concentration of serum folates for 2-week-old heifers markedly increased, while in 4-month-old heifers the effect of a supplement of pteroylmonoglutamic acid was less marked.
5. These results might indicate a need for pteroylmonoglutamic acid in young animals during the development of rumen function.