It is universally stated that antithyroid drugs are concentrated in human milk and are thus contraindicated during breast-feeding. We recently showed, however, that propylthiouracil (PTU) was not concentrated in milk and a study has now been made of the excretion of another widely used antithyroid drug carbimazole (CMI) in human milk. Methimazole (MMI) in blood and milk from five lactating women was measured after oral administration of CMI 40 mg, which is rapidly and completely transformed to the active antithyroid compound MMI. One hour after CMI, the mean serum-MMI reached 253 microgram/I and the mean concentration of MMI in milk reached 182 microgram/I. MMI was found to be unionized and not to be protein bound in serum, and it occurred in milk in the same concentration as the serum; the mean milk/serum ratio was 0.98. The mean total amount of MMI excreted in milk over 8 h was 34 microgram (SEM +/- 5, n = 5), i.e. 0.14% of the dose administered.