Laying hens were treated daily for 14 days with oral doses of [14C-phenoxy]cypermethrin (1.52 mg day−1, 0-7 mg kg−1) formulated on a small quantity of diet. Radioactivity in the eggs reached a plateau value of 0.05 μg equivalents g−1 8 days after the start of dosing. Most of the residue was found in the yolk and was a mixture of cypermethrin and material which was closely associated with neutral lipids and phosphatidyl cholines. Four and a half hours after the last dose, the birds were killed and selected tissues were taken for analysis. The highest residue was found in the liver. This was composed of cypermethrin and a mixture of very polar metabolites which were not hydrolysed to significant amounts of 3-phenoxybenzoic acid or its 4-hydroxy derivative.