1) Measurements of serum DDT concentration are capable of relfecting changes in oral DDT intake accurately and promptly. 2) Serum DDE level is only slightly increased by a 300-600 fold increase in DDT intake over a period of 5 months. It is therefore an essentially worthless measure of recent change in oral DDT intake. Stable individual differences in serum DDE levels, seemingly much wider than differences in dietary patterns (i.e. DDE intake), suggest the importance of individual metabolic factors in determining this level. 3) A high degree of correlation between serum DDT and DDE in day to day samples extracted once by hexane, probably reflects changes in extractability influencing the two measurements similarly.