The nonlinear transmission properties of C-60 and C-70 in toluene solution in the ultraviolet and visible spectral regions have been studied using nanosecond laser pulses. Whereas at 308 and 534 nm both fullerenes behave as reverse saturable absorbers, at 337 nm their behavior changes to that of a saturable absorber. A five-level rate equation model was developed that reproduces adequately the dependence of transmittance with intensity at the different irradiation wavelengths and allows us to estimate the absorption cross sections of the first excited singlet state for each fullerene.