Iodide concentration by the thyroid gland, an essential step for thyroid hormone synthesis, is mediated by the Na+/I- symporter (NIS). To identify factors that may regulate this process, we have studied NIS gene expression in the Fisher rat thyroid cell line (FRTL-5) by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) technique. Increasing concentrations of bovine TSH (0.1, 1, 10, 50 and 100 mU/l), with or without tumour necrosis factor-alpha (TNF alpha), interferon-gamma (IFN gamma) or interleukin-1 alpha (IL-1 alpha) were added to FRTL-5 cells previously deprived of TSH for a minimum of 5 days. RNA was extracted and samples were studied for NIS expression. TSH enhanced NIS mRNA expression in a dose-dependent manner, with induction evident at 0.1 mU/l, reaching a peak at 50 mU/l, an effect detected after 6 h of stimulation, but not in the first 2 h. Both TNF alpha and, to a lesser extent, IL-1 alpha inhibited basal and TSH-induced NIS expression. High concentrations of IFN gamma also downregulated TSH-stimulated NIS mRNA expression. Using the same technique, we also investigated NIS mRNA tissue distribution in two male and one female Wistar rats. High levels of NIS expression were detected in the thyroid, stomach, and mammary gland, lower levels were found in the intestine, adipose tissue and liver, borderline levels were expressed in the salivary gland, and no expression was detected in the kidneys. In summary, we have shown that TSH upregulates rat NIS gene expression in vitro, and this induction can be modulated by cytokines. Analysis of the distribution of rat NIS mRNA ex vivo demonstrated variable levels of NIS transcription in different tissue samples.