We examined interactions between base excision repair (BER) DNA intermediates and purified human BER enzymes, DNA polymerase beta (pol beta), apurinic/apyrimidinic endonuclease (APE1) and poly(ADP-ribose) polymerase-1 (PARP-1). Studies under steady-state conditions with purified BER enzymes and BER substrates have already demonstrated interplay between these BER enzymes that is sensitive to the respective concentrations of each enzyme. Therefore, in this study, using conditions of enzyme excess over substrate DNA, we further examine the question of interplay between BER enzymes on BER intermediates. The results reveal several important differences compared with data obtained using steady-state assays. Excess PARP-1 antagonizes the action of pol beta, producing a complete block of long patch BER strand-displacement DNA synthesis. Surprisingly, an excess of APE1 stimulates strand-displacement DNA synthesis by pol beta, but this effect is blocked by PARP-1. The APE1 exonuclease function appears to be modulated by the other BER proteins. Excess APE1 over pol beta may allow APE1 to perform both exonuclease function and stimulation of strand-displacement DNA synthesis by pol beta. This enables pol beta to mediate long patch sub-pathway. These results indicate that differences in the stoichiometry of BER enzymes may regulate BER.