Previous reports indicated that nitric oxide (NO) inhibits the iodide uptake in several species and the thyroperoxidase (TPO) mRNA expression in FRTL-5 cells. This work was aimed to analyse the role of the guanylyl cyclase/cyclic GMP (cGMP) pathway in the inhibitory effect of NO on iodide uptake and in the expression of TPO and thyroglobulin (TG) mRNA in TSH (500 m U/ml)-stimulated FRTL-5. Sodium Nitroprusside (SNP), a NO donor, increased cGMP levels (RIA) in a concentration and time-dependent manner. The cAMP level (RIA) decreased after 48 h of incubation with 500 m M SNP and was not modified at shorter times and lower SNP concentrations. An increase of cGMP was induced by TSH. The inhibition exerted by SNP or 8-Br-cGMP for 24 h on the iodide uptake (¹³¹I) was reversed by KT-5823 (1-10 m M), an inhibitor of the cGMP-dependent protein kinase (PKG) (Table 2). The TPO and TG mRNA expression (Northern-blot) was significantly reduced by 8-Br-cGMP. Data (24 h) were (Absorbance ratio mRNA/18 S rRNA, mean± SEM, arbitrary units): Control (C): TPO= 2.00, TG= 2.00; 8-Br-cGMP 100 m M: TPO= 1.97± 0.24, TG= 1.96± 0.03; 300 m M: TPO= 1.56± 0.13, TG= 1.23± 0.16**; 1000 m M: TPO= 1.22± 0.29*, TG= 1.01± 0.26** (*P< 0.05, **P<0.01 vs C; n=3, ANOVA, Student-Newman-Keuls test). In conclusion, an activation of PKG seems to be involved in the NO/cGMP-induced inhibition of iodide uptake in FRTL-5. A reduction in the cAMP production could be relevant in the action of high NO/cGMP levels. Evidence is provided for a role of the cGMP pathway in the regulation of thyroid specific genes.