The excretion and metabolism of oral 14C-pyridostigmine in the rat.

1. Pyridostigmine labelled with carbon-14 in the methyl group of the quaternary nitrogen has been used to investigate the excretion and metabolism of the drug after administration of single doses (500 mug) to the rat by stomach tube.2. Pyridostigmine is slowly excreted in the urine; the maximum excretion occurs between 1-3 hr after administration. In 24 hr 42% of the dose is excreted in urine and 38.4% is present in faeces and intestinal contents.3. The peak concentration of radioactivity in liver and blood occurs about 2 hr after administration.4. About 75% of the radioactivity in urine is present as unchanged pyridostigmine, the remainder as metabolite.5. The results are compared with those previously obtained after oral administration of neostigmine.6. It is concluded that after oral administration the absorption of pyridostigmine is greater and the metabolism substantially less than that of neostigmine.