Serum total fatty acids were estimated in controls and women using the oral contraceptives Eugynon (containing 0.5 mg.of norgestron and 0.05 mg ethinyl oestradiol) and Lyndiol (containing 2.5 mg. of linostrol and 0.05 mg of ethiny oestradiol) for periods ranging from 8 to 110 weeks. The serum total fatty acids level was found to be approximately 70% higher in the women using oral contraceptives, the difference being predominantly in the myristic and palmitoleic acid contents respectively. No significant difference was seen between the effect of Eugynon and Lyndiol usage upon blood fatty acids.