The anti-diabetic biguanide drugs metformin (METF) and phenformin (PHEN) may have anti-cancer effects. Biguanides suppress plasma growth factors, but nonetheless, the view that these mitochondrial inhibitors accumulate in tumor tissue to an extent that leads to severe energetic stress or alleviation of hypoxia-induced radioresistance is gaining ground. Our cell studies confirm that biguanides inhibits cell proliferation by targeting respiration, but only at highly suprapharmacological concentrations due to low drug retention. Biodistribution/PET studies of 11C-labeled metformin (11C-METF) revealed that plasma bioavailability remained well below concentrations with metabolic/anti-proliferative in vitro effects, following a high oral dose. Intraperitoneal administration resulted in higher drug concentrations, which affected metabolism in normal organs with high METF uptake (e.g., kidneys), but tumor drug retention peaked at low levels comparable to plasma levels and hypoxia was unaffected. Prolonged intraperitoneal treatment reduced tumor growth in two tumor models, however, the response did not reflect in vitro drug sensitivity, and tumor metabolism and hypoxia was unaffected. Our results do not support that direct inhibition of tumor cell respiration is responsible for reduced tumor growth, but future studies using 11C-METF-PET are warranted, preferably in neoplasia's originating from tissue with high drug transport capacity, to investigate the controversial idea of direct targeting.