It has been proposed that exogen agents could be related to the pathogeny of thyroid diseases through the activation of intrathyroidal macrophages. The nitric oxide (NO) has been reported to inhibit the iodide uptake in thyroid cells. Our goal was to analyze the influence of activated macrophages in culture on thyroid function in the rat thyroid cells FRTL-5 and the possible involvement of the NO. The TSH-stimulated FRTL-5 cells were cocultured with previously lipopolysaccharide (LPS)-activated (24h) rat peritoneal macrophages or incubated with the conditioned medium from the activated macrophages. The FRTL-5 ¹³¹I-uptake was inhibited when cocultured (24h) with LPS (2, 20, 100 µg/ml)-activated macrophages: % reduction vs control (TSH-stimulated cells) 34± 7*; 41± 5* and 29± 3*, respectively (n=7). The conditioned medium from activated macrophages induced a higher inhibition on iodide uptake (24h): 55± 6*; 70± 1* and 57± 8* (% reduction), respectively (n=4) (*P<0.01). In the LPS-treated macrophages the NO production increased 2 to 9 times. When NO synthesis was partially (50%) blocked by 1mM Nw -nitro-_L-arginine methyl ester (L-NAME), the ¹³¹I-uptake was restored in about 40 to 60 % in the coculture system. The TSH (1mUI/ml)-stimulated (control) proliferation (³H-Thymidine incorporation, ³H-Td) in FRTL-5 (48h) was diminished by conditioned medium from LPS (20 and 100 µg/ml)-activated macrophages: 62± 4* and 56± 5* (% reduction), respectively (n=3) (*P<0.01). A weak non significant inhibition of ³H-Td, which was restored by 1 mM L-NAME treatment, was found in the coculture system. In parallel, the NO donor sodium nitroprusside (NPS) (50-500 µM) which produced a NO level similar to that of the activated macrophages, had an antiproliferative action stronger than the activated macrophages. The induction of apoptosis seems not to be the responsible for the decrease in DNA synthesis as shown by the absence of DNA fragmentation (electrophoresis in agarose gels) in FRTL-5 under both experimental conditions. It is possible to conclude that the activated macrophages is able to modify the thyroid cell function by involving, at least in part, a NO action. These findings support the contention that the macrophage activation could be related to pathogenic processes of the thyroid gland.