A theoretical analysis of the fine structure of the lowest vibronic transitions of NH+ is performed by means of ab initio evaluation of all required spin-orbit and spin-spin interactions. The vibronic band system of this molecule is particularly complicated due to the presence of a high-spin electronic state, a(4) Sigma(-), nearly degenerate to the ground X(2)II level. A procedure recently described for the efficient evaluation of spinorbit two-electron integrals in Cartesian coordinates is extended to spin-spin interactions. The spin-spin dipolar and contact couplings are then evaluated by means of multireference configuration interaction (MRCI) techniques. All required effective spin-orbit and spin-spin matrix elements are evaluated using for the orbital description a complete active valence space self-consistent-field (CASSCF) treatment. The matrix elements have been found sufficiently accurate, and all computed spin-rovibronic transition energies are reproduced within +/-2 cm(-1). Our procedure and techniques are believed to be useful for theoretical analyses of high-resolution spectral data of open shell molecules.