Configuration and conformation of nicotine and its protonated form have been studied by MO theoretically using EHT and CNDO/2 methods. The form with angle of rotation ${ heta};=;280^{circ}$ of pyrrolidine ring was shown to be most stable and this was interpreted in terms of conjugative, electrostatic and seric effects. It was predicted that protonation occurs on the nitrogen atom of pyridine ring and forms a ${pi}$-complex.