Quadratic electro-optic effects (dc or low frequency Kerr effect) of bacteriorhodopsin dispersed in dried gelatin thin films are examined in the near resonance region at three wavelengths : 633, 647, and 676 nm. The films show relatively large quadratic electro-optic effects compared to other molecular dispersed systems. The purple membrane is fixed within the polymerized gelatin matrix, and we show that the electronic contribution to gamma dominates over possible orientational contributions. At 676 nm, the quadratic electro-optic coefficient s(1133)(-omega;0,0,omega) is 6.7 x 10(-20)m(2)/V-2 and the third order nonlinear susceptibility ((3)(chi 1133))(-omega;0,0,omega) is 7.0 x 10(-13) cm(4) statCoulomb(-2), with both values obtained for a protein concentration of 6.9 x 10(18) cm(-3). The orientationally averaged second molecular hyperpolarizability [gamma(-omega;0,0,omega)] determined from the quadratic electro-optic coefficients at 676 nm assuming an Onsager ellipsoidal local field factor is (10.8+/-5.1) x 10(-32) cm(7) statCoulomb(-2) [(1.34+/-0.63) x 10(-56) F-3 m(4) C-2]. The [gamma(-omega;0,0,omega)] value increases roughly tenfold when the probe wavelength is decreased to 633 nm. The behavior of gamma(-omega;0,0,omega), when fit to a two-state model, predicts that gamma(-omega;0,0,omega) is strongly enhanced via type III processes. Thus, the magnitude of gamma(-omega;0,0,omega) is dominated by a term (Delta mu(10)(2)x mu(10)(2))/(omega(10)-omega)(3), where Delta mu(10) is the change in dipole moment, mu(10) is the transition moment, and omega(10) is the transition energy of the lowest-lying allowed B-1(mu)*(+)-like pi,pi* state. We calculate that Delta mu(10) is 12.8+/-1.2 D, in good agreement with previous Stark and two-photon experimental values. Time-dependent Hartree-Fock methods based on the MNDO Hamiltonian yield reasonable agreement with experiment, underestimating gamma(-omega;0,0,omega) by factors of only 2-4, with the error increasing as the frequency approaches resonance.