Photophysical parameters in the excited singlet states of a series of 1-aminonaphthalene derivatives in cyclohexane, 3-methylpentane, and acetonitrile have been determined by means of time-resolved and steady-state fluorometry and time-resolved thermal lensing technique with the aid of MO calculations to elucidate the mechanism of internal fluorescence quenching of 1-aminonaphthalenes observed in nonpolar media. It is revealed that (1) the radiationless processes in nonpolar solvents are mainly due to fast internal conversion (IC; Phi(ic) = 0.97, k(ic) = 8.1 x 10(9) s(-1) for N,N-dimethyl-1-aminonaphthalene (DMAN) in cyclohexane at 293 K), (2) the fast IC occurs predominantly in the compounds having a pretwisted and flexible amino group with respect to the naphthalene ring, and the rate remarkably depends on temperature (the activation energy for the temperature dependent IC process of DMAN in 3-methylpentane is 5.1 kcal mol(-1)), indicating that the IC process is linked with an internal twisting motion of the amino group in nonpolar solvents, and (3) for the pretwisted compounds the rate of the S-1-->T-1 intersystem crossing (ISC) is also enhanced both in nonpolar and polar solvents. On the basis of the rate parameters obtained, the IC and ISC mechanisms of 1-aminonaphthalenes are discussed together with the remarkable solvent effects on their relaxation processes.