Both absorption and fluorescence spectra of 4-[4-(dimethylamino) phenyl]pyridine (4-DAPP) were investigated in various solvents at atmospheric pressure as well as at high pressures (up to 500 MPa). The spectroscopic results are well explained by a steeper torsional potential curve and a more planar relaxed conformation in the S-1-excited state than in the ground state. It was found that the solvent viscosity retards this steric conformational change. The pressure effect on the dihedral angle at the ground state between dimethylaniline and pyridine groups was determined. It decreases with a rate of similar to 1 degrees/100 MPa.