An increase of Gi alpha-related pertussis toxin substrates has been observed in the failing myocardium. In order to quantify the protein expression of Gi alpha directly, we developed a fast radioimmunoassay using the iodinated synthetic peptide 125I-KENLKDCGLF. beta-adrenoceptors were studied with 125I-cyanopindolol binding for comparison. Immunoblot experiments using recombinant G-protein alpha-subunits showed that DS4 immunostained the G-protein alpha-subunits with a rank order of potency rGi alpha 1 = rGi alpha 2 > rGo alpha >> rGi alpha 3. The G-protein alpha-subunits recognized by DS4 in human ventricular membranes comigrated with rGi alpha 1 and rGi alpha 2. The radioimmunoassay had a sensitivity of 2.5 micrograms/ml transducin alpha with an interassay variation of less than 10%. The non-labelled peptide selectively competed with the myocardial 40 kDa membrane protein for binding to the antiserum DS4. Radioimmunochemical quantification of Gi alpha from cardiac membranes showed that in left ventricular membranes (LV) from dilated cardiomyopathy (DCM), there was an increase of Gi alpha by 138.5% when related to mg protein and 135% when related to 3H-ouabain binding sites as membrane marker. In LV from ischaemic cardiomyopathy (ICM), the increase was smaller (58.4%) when related to mg protein compared to the increase of Gi alpha when related to 3H-ouabain binding sites as membrane marker (155% v NF). In contrast, in the right ventricles (RV) there was no increase of Gi alpha in ICM or DCM. The numbers of beta-adrenoceptors were reduced in RV and LV of both, ICM and DCM. It is concluded that the radioimmunoassay may become an important tool for studying the expression of Gi alpha-protein levels and changes thereof in pathological conditions. The amount of immunodetectable Gi alpha-proteins is increased in failing LV due to DCM and ICM but not in RV, while beta-adrenoceptor down-regulation occurred in RV and LV in both conditions. These findings might indicate that the liability of the LV but not of RV to express Gi alpha-proteins may be increased in predominant LV heart failure. Alternatively, the underlying mechanism, e.g. sympathetic activation, may be regulated locally in the failing heart producing different changes in adjacent chambers.