Approximate self-consistent orbitals for excited electronic states of cis- and trans-1, 3-butadiene are obtained by a modification of Roothaan's procedure, in the non-empirical $\pi $-electron approximation. The integrals used were evaluated by Parr & Mulliken for calculation of the ground-state electronic wave function. The effects of configuration interaction are calculated by an approximate method and compared with an exact calculation. Molecular orbitals have been obtained both with and without the auxiliary condition that spatial factors of both $\alpha $ and $\beta $ spin-orbitals should be members of a single orthogonal set. Semi-empirical values for the basic integrals, due to Pariser & Parr, have also been used to calculate the energies of excited states by the approximate configuration interaction method. Energy levels derived from the Pariser-Parr integrals are in close agreement with observed levels, which differ considerably from those calculated from the Parr-Mulliken non-empirical integrals.