Macrocyclic pyrrolizidine alkaloids (PAs) are a mixed group of phytotoxins with similar chemical structures and varying biological effects. A commonly studied member of this group is senecionine (SEN) which causes hepatotoxicity. We have undertaken metabolism and excretion studies of SEN in rats to provide data for comparison between PAs and to evaluate the potential role of metabolism and excretion in toxicity. Following intravenous administration of [14C]SEN (60 mg/kg, 10 microCi/kg), bile, urine and blood were collected over a 7-h period. Of the total administered radioactivity, 44% and 43% were excreted in the bile and urine, respectively. Using mass spectroscopy, senecionine N-oxide (SENNOX) was identified as the major metabolite in bile (52% of 44%) and urine (30% of 43%). For the total 7 h, less than 5% in bile and 18% in urine was excreted as parent alkaloid. The plasma concentration of Senecionine-equivalents/g (SEN-EQ/g) decreased from 107 to 12 nmol, while red blood cell (RBC) concentrations declined from 109 to 26 nmol/g. Without bile collection, the plasma levels of SEN-EQ were similar, while the final RBC level was almost double (47 vs. 26 nmol/g) and total radioactivity excreted in the urine was increased (59% vs. 43%). Biliary pyrrolic metabolites were estimated to be 1.43 mg, using a dehydroretronectin standard.