G-protein-coupled receptors (GPCRs) are seven transmembrane-helix proteins that participate in transmitting signals across cell membranes to regulate multiple functions of the human body. Rhodopsin is an important prototype for studying class A GPCRs. Here we address aspects of the GPCR activation not available for X-ray crystallography due to cryogenic temperatures and the absence of the membrane environment [1,2]. Our hypothesis was that the activation process involves an allosteric mechanism where a dynamical ensemble of states is affected by protein-membrane interactions [1]. Rhodopsin was investigated in native disk membranes as well as POPC or DOPC lipids by FTIR and UV-visible spectroscopy. By conducting pH titrations for different spectroscopic bands at various temperatures we identified distributions of states in the activating equilibrium and determined equilibrium constants and thermodynamic parameters for the MI-to-MII transition and proton uptake [3]. Analysis of pH titration curves showed that temperature increase had a crucial effect on rhodopsin activation enabling coexistence of multiple inactive and active states. By changing the lipid environment, we validated that lipids with negative monolayer curvature facilitate rhodopsin activation by stabilizing the active Meta-II state [1]. Furthermore, we analyzed the distribution of pKa and alkaline end-point values in the whole range of FTIR spectra where changes were observed upon activation. The results for all temperatures and different lipid compositions showed that activation of rhodopsin yields a conformational ensemble of states with thermodynamic parameters corresponding to different tiers of an enthalpy-entropy folding funnel. We propose that formation of Meta-II involves a partial unfolding of the receptor that exposes recognition elements for the G-protein (transducin) that are buried in the dark state.