Mineralocorticoids and glucocorticoids are important regulators of electrolyte homeostasis and arterial blood pressure. Their effects are mediated by the mineralocorticoid (MR) and the glucocorticoid receptor (GR), respectively. The present study was designed to determine how the two isoforms of the human GR, the "classic" GR alpha and the non-hormone-binding GR beta, interfere with the transcriptional effects of the hormone-activated human MR. COS-7 monkey kidney cells were transfected with different mineralocorticoid-responsive reporter plasmids and a vector expressing the human MR protein. Different amounts of either control, GR alpha, or GR beta plasmid were co-transfected, and luciferase activity was measured after stimulation with aldosterone and/or dexamethasone. MR-mediated stimulation of transcription was enhanced by co-transfection of the GR alpha expression vector. In contrast, MR-mediated stimulation of transcription was strongly inhibited by co-transfection of equal amounts of the GR beta expression vector. Reverse transcription-polymerase chain reaction (RT-PCR) showed expression of both GR isoforms as well as of MR in the human kidney. These data indicate that the two isoforms of the human GR exert opposite effects on mineralocorticoid activity. We conclude that the ratio between GR alpha and GR beta can define the sensitivity of mineralocorticoid target tissues to aldosterone. Imbalances of this ratio may participate in clinical syndromes of impaired or augmented mineralocorticoid sensitivity, such as certain cases of pseudohypoaldosteronism or, possibly, primary arterial hypertension.