Suramin, a polysulfonated naphthylurea, is under investigation for the treatment of several cancers. It interferes with signal transduction through Gs, Gi, and Go, but structural and kinetic aspects of the molecular mechanism are not well understood. Here, we have investigated the influence of suramin on coupling of bovine rhodopsin to Gt, where G-protein activation and receptor structure can be monitored by spectroscopic in vitro assays. Gt fluorescence changes in response to rhodopsin-catalyzed nucleotide exchange reveal that suramin inhibits Gt activation by slowing down the rate of complex formation between metarhodopsin-II and Gt. The metarhodopsin-I/II photoproduct equilibrium, GTPase activity, and nucleotide uptake by Gt are unaffected. Attenuated total reflection Fourier transform infrared spectroscopy shows that the structure of rhodopsin, metarhodopsin-II, and the metarhodopsin-II Gt complex is also not altered. Instead, suramin dissociates Gt from disk membranes in the dark, whereas metarhodopsin-II Gt complexes are stable. Förster resonance energy transfer suggests a suramin-binding site near Trp207 on the Gtα subunit (Kd ∼0.5 μM). The kinetic analyses and the structural data are consistent with a specific perturbation by suramin of the membrane attachment site on Gtα. Disruption of membrane anchoring may contribute to some of the effects of suramin exerted on other G-proteins.