The total synthesis of a 5,15-di[3,5-(o-carboranylmethyl)phenyl]porphyrin 5, its zinc(II) complex 6, and the corresponding nido-carboranylporphyrins 7 and 8 are reported. The molecular structures of porphyrin 6 and of potassium nido-carborane were obtained and are described. The biodistribution of nido-carboranylporphyrins 7 and 8 in BALB/c mice bearing EMT-6 mammary tumors are presented and compared. Both compounds are effective tumor localizers and delivered therapeutic concentrations of boron to tumors (mean+/-standard deviation): 32.5+/-7.1 and 54.3+/-14 microg/g for 7 and 8, respectively, 2 days after the last of 3 injections of a total boron dose of 23 mg/kg body weight. The zinc(II) complex 8 was found to deliver 1.2-1.7 times higher amounts of boron to tumors than 7, with lower tumor-to-blood boron concentration ratios (9.8/1 and 4.7/1 for 7 and 8, respectively, 2 days after injections). The tumor-to-brain boron concentration ratios were >100/1 for both porphyrins 2 days after administration. Both nido-carboranylporphyrins 7 and 8 were well-tolerated at the concentrations used (75 and 78 mg/kg body weight, respectively) and no morbidity or mortality were observed in these studies.