Placental Transfer and Pharmacokinetics of a Single Dermal Dose of [14C]Methyl Parathion in Rats

The pharmacokinetics and placental transfer of a single dermal 10.0 mg (10mCi)/kg dose of uniformly phenyl-labeled [14C] methyl parathion (0,0-dimethyl 0-4-nitrophenyl phosphorothioate) were in-vestigated in pregnant Sprague-Dawley rats at 14–18 days of gesta-tion. Three rats were killed at each time interval: 1, 2, 4, 12, 24, 48, 72, and 96 h after dosing. Radioactivity disappeared biexponentially from the administration sites, which retained 50 % and 3 % of the dose after 1 h and 96 h, respectively. Most of the absorbed radioactivity was excreted in the urine (91%). Only 3 % of the 14C was recovered in the feces. One h after the administration, radioactivity was detected in all tissues, including fetal tissue. The peak maternal plasma con-centration of radioactivity (ng methyl parathion equivalent/ml) was 1005 at 2 h, compared to 318 ng for fetal plasma at 12 h. The maximum concentrations of radioactivity (ng methyl parathion equivalent/g), detected in most tissues within 12 h of dosing, were, i