The effect of oral contraceptive steroids on drug‐metabolizing capacity was determined by the use of plasma antipyrine and phenylbutazone half‐lifes as indices. Drug‐metabolizing capacity seemed to be impaired in women taking oral contraceptive steroids since the mean plasma antipyrine half‐life in female control subjects was 10.8 ± 2.4 hours (N = 36) while that in the “pill” group was 14.1 ± 3.1 hours (N = 26). The difference between the groups was significant (p < 0.001). With phenylbutazone, the mean control value was 70.6 ± 30.1 hours (N = 18) and that for the subjects taking oral contraceptives 80.2 ± 30.6 hours (N = 18). In this case the difference was not statistically significant (p > 0.05).