We report measurements of the third-order optical susceptibilities of 13-cis and all-trans retinal derivatives in chloroform liquid solutions using the Z scan and the optical Kerr gate techniques, with laser pulses of 70 ps and 60 fs, at 532 and 800 nm, respectively. The corresponding second hyperpolarizabilities of those molecules are estimated and the results compared to those obtained from semiempirical time-dependent Hartree-Fock calculations. The observed trends for the values of the nonlinear response are explained in terms of the pi electron delocalization along the polyenic chain of the molecules.