To determine the effect of oral administration of thyrotropin-releasing hormone (TRH) on the thyroid function and on the composition of breast milk in the early puerperium, six lactating women were treated with a single dose of 40 mg of synthetic TRH and six women were treated with placebo. Serial serum samples taken before and between one and 25 hours after TRH administration were assayed with specific radioimmunoassays for thyrotropin (TSH), triiodothyronine (T3) and total thyroxine (T4). Milk samples were collected three times a day and their major fatty acids were determined by gas-liquid chromatography and were compared with those obtained from normal lactating women. A statistically significant TSH elevation was observed between one and six hours after TRH administration, with a peak value of 23.7 +/- 10.6 mU/liter at three hours. The T3 concentration rose between three and nine hours after TRH administration, with a peak of 6.3 +/- 1.2 nmole/liter at six hours. The T4 elevation was statistically significant between six and 12 hours after TRH administration. The fatty acid content of milk samples from women treated with TRH did not differ from the normal series. A single daily dose of oral TRH thus caused a temporary thyroid stimulation. It is doubtful whether this could lead to hyperthyroidism since the levels of thyroid hormones became normal within ten hours after TRH administration.