A number of 9H-indeno[2,1-c]pyridazine N-oxides (3a-c) and benzo[f]cinnoline N-oxides (4,5a-c) have been synthesized and tested for antimicrobial activity. All new products were inactive against Gram negative bacteria and fungi. In contrast, among the compounds synthesized, 3b, 4b and 5b showed a moderate activity against Gram positive Staphylococcus aureus and Staphylococcus epidermidis. Of the present series, the 9-nitro-benzo[f]cinnoline N-oxide 5b possessed the highest activity especially against Trichomonas vaginalis (MIC = 3.9 micrograms/ml).