Oral glucose-tolerance tests were performed and fasting serum cholesterol and triglyceride levels measured in 1628 Caucasian women taking combined oestrogen/progestagen oral contraceptives (o.c.) and 577 women not taking O.C. The former were divided into six groups according to the composition of the O.C Glucose tolerance deteriorated in all O.C. groups containing oestrane progestagens (nortestosterone-derived) or the gonane, norgestrel, but was unaltered by O.C. containing a pregnane progestagen (derived from progesterone). The greatest deterioration was with O.C. containing 75 microgram or more oestrogen, and this was associated with impairment of the early insulin response to glucose. In O.C. containing a pregnane progestagen insulin secretion was unaffected. In the remaining O.C. groups insulin secretion was increased; this was most pronounced with the O.C. containing a gonane progestagen. Serum-cholesterol was elevated only with O.C. containing 75 microgram or more oestrogen and an oestrane progestagen and tended to be lower in O.C. containing a gonane progestagen. O.C.-induced hypertriglyceridaemia was oestrogen-dose-related, and this effect was potentiated by the pregnane progestagen. The gonane progestagen antagonised oestrogen-induced hypertriglyceridaemia.