The cobalt atom of hydroxocobalamin (OHC) binds cyanide and nitric oxide (NO) and OHC attenuates vascular responses to NO in vitro. NO mediates the hypotension of endotoxemia. Thus, we tested the postulate that OHC may attenuate the acute phase hypotension and toxicity associated with administration of Escherichia coli endotoxin (LPS). Rats were given OHC (20 mg/kg i.v.) or phosphate-buffered saline (PBS, 1 ml/kg i.v.) 30 min before or 15 min after giving LPS (0.8 mg/kg i.v.). Administration of OHC to PBS-treated control rats did not affect mean arterial pressure (MAP), heart rate or the plasma or urine content of the reactive nitrogen intermediates nitrate and nitrite (RNI). LPS decreased MAP by 50 mm Hg in PBS-treated rats and increased the plasma and urinary content of RNI. Administration of OHC to PBS-treated rats did not affect MAP or RNI. However, treatment with OHC before or after giving LPS attenuated LPS-induced hypotension and increases in plasma RNI and enhanced LPS-induced urinary excretion of RNI. OHC (20 mg/kg i.p.) or cyanocobalamin (10 mg/kg i.p.) given to Swiss-Webster mice 30 min before giving LPS (16 mg/kg i.p.) decreased the 24-hr mortality of LPS from 80 to 50% and the 36- and 96-hr mortality from 100 to 60% (OHC) or 70% (cyanocobalamin). Urine obtained from conscious rats given LPS (5 mg/kg i.p.) and OHC (20 mg/kg i.p.) exhibited a UV-visible absorbance spectrum with absorbance peaks characteristic of that formed after coincubation of NO and OHC.(ABSTRACT TRUNCATED AT 250 WORDS)