Placental transfer of pyridostigmine in the rat.

1. Carbon-14 labelled pyridostigmine in single doses was given by intramuscular injection to pregnant rats, and maternal blood, foetal and placental homogenates were examined for total radioactive content, pyridostigmine and its metabolite, 3-hydroxy-N-methyl pyridinium.2. The level of radioactivity in all tissues was highest at 15 min and fell to negligible amounts at 24 hr. The concentrations in placenta were similar to those in maternal blood, but there was some indication of retention of radioactive drug in the placenta. The concentrations in the foetus were substantially lower, but the subsequent slow decline in concentration suggests that cumulation of the drug could occur with repeated doses.3. The percentage of metabolite in the foetus was considerably higher than in placenta and blood, and the possible reasons for this are discussed.