Reaction centers of Rhodobacter sphaeroides undergo a approximately 20 A3/mole volume contraction in < 50 ns after excitation. The rapid volume change is tentatively assigned to electrostriction. From its magnitude, we infer that the effective dielectric coefficient is 10-15 if the compressibility of the reaction center is similar to that of globular proteins. The volume contraction is not sensitive to replacement of the natural ubiquinone at the QA site by other quinones or to the occupancy of the QB site. The quenching caused by pressure on the reaction centers most likely occurs on a faster time scale than that of electron transfer.