Spin density distributions and isotropic hyperfine coupling constants of N@C-60, P@C-60, N@C-70, N@C-60(CH2)(6), and N@C-60(SiH2)(6) are theoretically investigated by testing B3LYP and MP2 methods for the free N and P atoms. When N is encapsulated inside C-60, the calculated spin density at nucleus N increases. In P@C-60, the corresponding increase is much larger. N@C-60 has a slightly larger spin density at N than N@C-70. These agree well with the observed isotropic hyperfine coupling constants. Upon addition of CH, groups on the N@C-60 cage, the spin density at N is slightly decreased. However, addition of SiH2 groups increases significantly the spin density at the nitrogen nucleus. (C) 2003 Elsevier B.V. All rights reserved.