Membranous adenylyl cyclase (AC) subtypes play differential roles in the regulation of cell functions. The C1- and C2-subunits of AC form a heterodimer that efficiently catalyzes cAMP formation and constitutes a very useful model system for AC analysis at a molecular level. Intriguingly, C1 and C2 homodimers exist, too. The C2 homodimer is catalytically inactive and possesses two forskolin binding sites. However, little is known about the C1 homodimer. Therefore, in this study, we examined the C1 homodimer. C1 exhibited exceedingly low catalytic activity but high substrate-affinity. Fluorescence studies with the AC inhibitor 2',3'-O-(2,4,6-trinitrophenyl)-ATP suggested that 2 mol of C1 binds 1 mol of nucleotide, pointing to homodimerization. C1 also bound the AC inhibitor 2',3'-O-(N-methylanthraniloyl)-GTP as assessed by direct fluorescence and fluorescence resonance energy transfer studies. Molecular modelling revealed that in the C1 homodimer, the catalytic base arginine is exchanged against histidine. The lower basicity and shorter side chain of histidine probably account for the low catalytic activity. In conclusion, the C1 homodimer of AC binds nucleotides with high affinity, but exhibits only exceedingly low catalytic activity. The low catalytic activity of the C1 homodimer may constitute a mechanism by which in intact cells dimeric AC molecules exhibit a high signal-to-noise ratio upon stimulation by receptor agonists.