The molecular structure of nicotine was determined by means of gas electron diffraction. The nozzle temperature was about 116 degreesC. The results of RHF, MP2, and DFT calculations were used as supporting information. The electron diffraction data were well reproduced by assuming the mixture of two conformers where both the methyl group and the pyridine ring are in the equatorial positions of the pyrrolidine ring. The determined structural parameters (r(g) and angle(alpha)) are as follows: = 1.462(4) Angstrom; = 1.541(4) Angstrom; = 1.345(2) Angstrom; = 1.397 Angstrom (d.p.); r(C-pyrrol-C-pyrid) = 1.502 Angstrom (d.p.); = 1.116(4) Angstrom; = 1. 106 Angstrom (d.p.); angleC-N-C-pyrrol = 108.4(15)degrees; = 102.4degrees (d.p.); = 108.2(33)degrees; angleC-N-C-pyrid = 116.8(2)degrees; = 124.1degrees (d.p.); = 118.3degrees (d.p.); angleN(pyrrol)-C-pyrrol-C-pyrid = 116.8(10)degrees; angleC(pyrrol)-C-pyrrol-C-pyrid = 115.8(20)degrees; = 111.9(41)degrees; alpha = 45.7(25)degrees; beta = 32.9(38)degrees; phi = -87.7(74)degrees. Angle brackets denote averaged values, values in parentheses are 3 times the estimated standard errors referring to the last significant digit, and d.p. denotes dependent parameters. Angle alpha is the methyl out-of-plane angle, and P is the puckering angle of the pyrrolidine ring. Angle 46 is the dihedral angle between the planes of pyrrolidine and pyridine rings. The distances between the two N atoms are determined to be 4.885 +/- 0.006 and 4.275 +/- 0.007 Angstrom for the two conformers. By comparing these values with the distance between the N and carbonyl 0 atoms of acetylcholine, a strong nicotinic agonist, it is concluded that among the two conformers of nicotine, only the one with the longer N N distance has the nicotinic activity.