Plasma and red blood cell (RBC) folic acid levels, as well as plasma vitamin B₁₂ levels were determined in Vervet monkeys (Cercopithecus aethiops). All the vervets were apparently healthy and without symptoms or lesions typical of folic acid and/or vitamin B₁₂ deficiencies. Competitive protein binding radioassays were used to determine folate and vitamin B₁₂ values in animals fed 4 different diets. The B₁₂ levels for all the groups ranged between 866 and 5867 pg/ml and showed an inverse relationship with the FA measurements. The lowest mean RBC folic acid content in a group fed an atherogenic diet for 3 years was 12·8 ng/ml. For the other 3 diets, mean RBC folic acid levels were 90·7, 132·3 and 152·8 ng/ml, respectively.

A megadose of 25·6 mg of folic acid per day for 99 days was given to 3 adult males. No obvious toxic effects were observed in these animals although mean RBC folic acid levels increased to 1013 ng/ml.