Oxazolidinones are a novel class of synthetic antibacterial agents active against gram-positive organisms including methicillin-resistant Staphylococcus aureus as well as selected anaerobic organisms. Important representatives of this class include the morpholine derivative linezolid 2, which is currently in phase III clinical trials, and the piperazine derivative eperezolid 3. As part of an investigation of the structure-activity relationships of structurally related oxazolidinones, we have prepared and evaluated the antibacterial properties of a series of piperazinyl oxazolidinones in which the distal nitrogen of the piperazinyl ring is substituted with a six-membered heteroaromatic ring. Compounds having MIC values </= 2 microg/mL vs selected gram-positive pathogens were discovered among each of the pyridine, pyridazine, and pyrimidine structural classes. Among these the cyanopyridine 17, the pyridazines 25 and 26, and the pyrimidine 31 exhibited in vivo potency vs S. aureus comparable to that of linezolid.