This study was aimed at developing ligands to evaluate tumor hypoxia by planar scintigraphy. Two 2-nitroimidazole analogues were developed as precursor compounds to image hypoxic tumors. Both tosylmisonidazole (Ts MISO) and tosylerythronitroimidazole (Ts ETNIM) were labeled with 131I. The biodistribution and autoradiographic evaluations by planar scintigraphy of 131I-IMISO and 131I-IETNIM were conducted at 1, 2 and 4 hours after administration to rats bearing 13762 breast tumors. Biodistribution of 131I-IMISO was also evaluated in Madison lung tumor-bearing mice. Intratumoral oxygen tension was measured by the Eppendorf system. Biodistribution showed similar tumor/blood and tumor/muscle count density ratios for both compounds. The thyroid uptake of both analogues was increased with time, suggesting in vivo deiodination probably occurred. Autoradiographs of 131I-IMISO and 131I-IETNIM revealed good visualization of the neoplasms. The tumor oxygen tension was 3-6 mmHg as compared to the normal tissue oxygenation of 30-40 mmHg. The findings indicate that these analogues can localize in the hypoxic region of solid tumors and may assist with quantitation of the hypoxic fraction of tumor for proper selection and evaluation of appropriate radiotherapy and chemotherapy.