The fraction of injected activity that was excreted through the breast milk of nursing mothers at different times after the injection of various radiopharmaceuticals has been measured in 21 patients. For 99mTc-labeled radiopharmaceuticals the total excreted fraction was 10% for pertechnetate and 1.5-3% for MAA, plasmin, diethylenetriaminepentaacetic acid (DTPA), and methylene diphosphonate (MDP). For [125I]hippuran and [131I]hippuran the corresponding value was 3%. For the above mentioned radiopharmaceuticals the activity concentration in the milk decreased exponentially with an effective half-life of approximately 4 hr. For chromium-51 ethylenediaminetetraacetic acid ([51Cr]EDTA) and [99mTc]RBC, much smaller amounts were excreted in the breast milk. The absorbed dose to various organs of the baby has been calculated. We conclude that when [99mTc]pertechnetate, [99mTc]MAA, [99mTc]plasmin, [125I]hippuran, or [131I]hippuran are used the child should be fed just before the administration of the radionuclide to the mother and the next three milk fractions should not be used. For [99mTc]DTPA and [99mTc]MDP as well as [51Cr]EDTA, only the first fraction should not be used. According to our earlier investigations breast feeding has to be stopped for at least 3 wk after investigations with [125I]fibrinogen.