6-Phenyl analogs of toxoflavin (1-methyl-6-phenylpyrimido[5,4-e]-1,2,4-triazine-5,7(1H,6H)-diones ) (7a--f) and their 4-oxides (8a-f) were synthesized by nitrosative or nitrative cyclization of the aldehyde hydrazones (6a-f) of 6-(1-methylhydrazino)-3-phenyluracil (5). Both sets of compounds, 7a-f and 8a-f, gave the corresponding 1-demethyl derivatives (10a-f) upon treatment with nucleophiles such as dimethylformamide (DMF) and acetic acid under heating. The activities of toxoflavins (1a-e), toxoflavin 4-oxides (3a-e) and their 6-phenyl analogs (7a-f and 8a-f) against a variety of bacterial and fungal strains were examined. Most of the compounds showed strong inhibitory activities against gram-positive bacteria. Among the compounds, 1c, 1d, 1e, and 3c exhibited the strongest inhibitions of Micrococcus lutea (0.5 micrograms/ml minimal growth-inhibitory concentration) and Staphylococcus aureus 4R (1 microgram/ml), as well as Bacillus subtilis and Staphylococcus aureus (1-2 micrograms/ml). Most of the compounds had strong antifungal activity (2-100 micrograms/ml minimal growth-inhibitory concentration) against Candida albicans and Saccharomyces cerevisiae.