Abstract Patients taking anticonvulsants such as phenobarbitone, phenytoin and carbamazepine together with their oral contraceptive steroid may suffer contraceptive failure because of the enzyme-inducing properties of these anticonvulsants. We have examined, in six women, the effect of sodium valproate, an effective broad spectrum anticonvulsant, on the area under the plasma concentration versus time profile (AUC) of ethinyloestradiol (EE 2) and levonorgestrel (Ng). Prior to sodium valproate therapy the mean AUC for EE 2 was 880 ± 109 pg/ml × h (± S.E.) and for levonorgestrel it was 29.1 + 2.9 ng/ml × h (n = 4). Between two and four months after sodium valproate therapy the mean AUC figures had not changed significantly, the figure for EE 2 being 977 ± 130 pg/ml × h and for levonorgestrel 29.2 ± 1.9 ng/ml × h (p ⩾ 0.1 in each case). We conclude that sodium valproate in the dose used (200mg b.d.) does not interact with oral contraceptive steroids.