Ultraviolet photoelectron spectroscopy (UV PES) and CNDO/s molecular orbital calculations have been employed to investigate the electronic structure of clonidine and some other 2-(phenylimino)imidazolidines. The assignment of the bands in the spectra to particular molecular orbitals is based on the CNDO/s results in conjunction with Koopmans' theorem, substituent effects, and differences in intensity between the He (I) and He(II) spectra. The location of the energy levels of orbitals with mainly nN and delta character is not correctly estimated estimated by CNDO/s, while the pi orbital energy levels are satisfactorally predicted. The UV PES and CNDO/s results indicate in contrast investigated 2-(phenylimino)imidazolidines, which may indicate that differences in hypotensive activity cannot be ascribed to variations in steric hindrance within the molecules. The first ionization energies of the pharmacologically active 20-(phenylimino)imidazolidines do not correlate with hypotensive activity based on dosage data after intravenous administration to rats.