Summary The absolute conformation and the reactivity of atoms of the neutral form of natural tetrahydrobiopterin [(6R)-6-(L-erythro-1 ′,2′ -dihydroxypropyl)-5.6.7.8-tetrahydropterin] was determined by the Molecular-Orbital method within the MINDO/3 framework. Two different conformations (Form A and Form B) of the alkyl side chain relative to the pterin ring were obtained as the optimized form for the lowest energy conformation (ΔH= -174 kcal/lmol) of this molecular constitution with the same molecular width of 11.2 Å. Form A and B were distinguishable in the dihedral [01′ -C1′ -C6-H6J of - 88° and 30°. respectively. Net charges of atoms of Form A and B suggested that the predominant reactivity of the atoms of N1 C2. C4. O4, C4a. and C8a of the pterin ring and C1′. C2′. O1′ and O2′ of the side chain. It was also suggested that the position 2' is more active than the position 1′. in the alkyl side chain of Form A and B. toward NADP+ in the reverse reaction of sepiapterin reductase.