Direct measurements of the binding between light-activated rhodopsin (Rho*) and transducin, the retinal rod G-protein, revealed a strongly cooperative interaction. Cooperativity was assessed by measuring the association of 125I-labeled transducin (Gt) to Rho* in urea-stripped rod outer segment membranes at equilibrium. Analysis of 125I-Gt binding curves gave a Hill coefficient of 1.8. These data were consistent with a two-site model in which binding of the first 125I-Gt to Rho* increased the binding of the second 125I-Gt approximately 40-fold (Kd values were 80 +/- 30 and 1.9 +/- 0.7 nM, respectively). The effects of GDP on the binding were also investigated. GDP decreased the affinity between Rho* and Gt approximately 100-fold but did not decrease the degree of cooperativity. Binding curves of 125I-Gt in the presence of 1 mM GDP showed a Hill coefficient of 1.9. The data were also consistent with a two-binding site model in which binding of the first 125I-Gt increased the binding of the second 125I-Gt approximately 70-fold (Kd values were 13.7 +/- 5.4 and 0.20 +/- 0.08 microM, respectively). The Gt alpha subunit in the absence of Gt beta gamma also bound Rho* in a cooperative manner. These data implicate a role for the cooperative association of Rho* and Gt in the light activation cascade of retinal rods.