A series of imidazo [1,2-a] quinoxaline mono-N-oxides and their aza- analogues have been synthesized together with analogues substituted in the 8-position. These compounds have been evaluated as bioreductively activated cytotoxins in vitro and in vivo. These compounds had differential cytotoxicities in vitro of up to 20 for 8-amino derivatives such as RB90740 and 65 for 8-aminoalkoxy derivatives such as 1,2-dihydro-8-(1-(demethylamino)ethoxy)-4-phenylimidazo [1,2-a] pyrido [3,2-e] pyrazine 5-oxide, but were disappointing in vivo with a maximum growth delay of 10 days compared with 30 days for SR4233 in the RIF-1 tumor model. RB90740 is only effective at killing V79 cells at extremely low levels of oxygen, in contrast to SR4233, and this oxygen dependence can explain the poor and often variable activity of the compound in vivo. 1,2-dihydro-8-(1-(demethylamino)ethoxy)-4-phenylmidazo [1,2-a] pyrido [3,2-e] pyrazine 5-oxide, as the most effective drug in vitro, remains the lead structure for any further drug development.