Light activates a 3',5'-cyclic GMP phosphodiesterase (PDE) in bovine retinal rod outer segments. The light is absorbed by rhodopsin situated in the disk membranes. PDE is a three-subunit peripheral protein on the disks and appears to be activated via a guanine nucleotide binding protein (G) in the presence of activated rhodopsin and GTP. Does the activation occur by collision coupling of G and PDE? We have studied the protein-protein interactions of PDE in situ in disk membranes by radiation inactivation. Irradiation of a protein with high-energy electrons leads to loss of activity in proportion to radiation dose and the molecular weight of the protein. We see no change in the size of PDE upon activation by light and 100 microM guanosine 5'-(beta, gamma-imidotriphosphate) (Gpp[NH]p) compared with PDE in dark with 260 microM GTP. Application of statistics to our data shows that a 27 000 change in molecular weight would be significant at the 95% level but that smaller changes would go undetected. The apparent molecular weight is 176 000 +/- 27 000 (mean +/- 95% confidence limit), in agreement with the size determined by polyacrylamide gel electrophoresis. Thus there appears to be either (i) no permanent change in PDE size on activation or (ii) a small change, undetectable by the technique, or (iii) an exchange of subunits such that no net change in molecular weight is seen.