Since, in the presence of ANF, urinary cGMP was shown to be of glomerular origin, a possible paracrine effect of luminal cGMP on the medullary thick ascending limb (mTAL) function was investigated. Net chloride reabsorption (JCl) was determined on isolated microperfused tubules from mouse kidney. Addition of 10(-6) M cGMP to the lumen significantly decreased JCl by 46.5 +/- 4.6%. A concentration-dependent decrease of the transepithelial voltage was observed, with a 10(-8) M threshold. Added to the bath, ANF (10(-7) M) as well as urodilatin (6 x 10(-8) M) decreased JCl by 29.8 +/- 3.9% and 36.9 +/- 5.1%, respectively, an effect reproduced by 8-bromo cGMP and associated with a significant increase in tubular cGMP content. The inhibitory effect of ANF was similar whether or not cGMP was present in the lumen. Furthermore, increasing intracellular cGMP content by 8-bromo cGMP did not prevent a further effect of luminal cGMP. Finally, H-8, which blocked the effect of ANF, urodilatin, and 8-bromo cGMP, failed to abolish the luminal cGMP-induced decrease of JCl, suggesting that this effect did not require a cGMP-dependent protein kinase activation. It is concluded that luminal cGMP inhibits the reabsorptive function of the mTAL through a pathway different from the intracellular cGMP production.