Although radioactive iodide uptake (RAIU) is one of the reliable diagnostic methods for thyroid function in adult humans, especially in the diagnosis of thyrotoxicosis, there are limited data for RAIU during pregnancy and lactation in humans and animals. Therefore, we proposed to validate RAIU for the lactating rhesus monkey to further human model studies in thyroid disease. RAIU was performed at 6 and 24 h using 100 mu Ci of (123)I orally in four lactating monkeys. The thyroid and thigh were counted using a scintillation probe and multichannel analyser. A dose/standard ratio of counts/minute was calculated to compensate for background, utilizing differences in the activity between the dose administered and a standard. Thyroidal RAIU varied significantly among monkeys: 6.71 +/- 2.40% for the 6 h uptake and 15.44 +/- 7.71% for the 24 h uptake. These data showed that the RAIU test may allow a rational clinical approach to thyroid function testing for lactating rhesus monkeys. Additional studies are needed for assessing thyroid function in rhesus monkeys of varying ages and gender with clinical abnormalities.