Radiolabelled racemic cypermethrin, and the cis and trans isomers of 3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylic acid (labelled at C-1 of the cyclopropane ring) were each separately administered orally in feed for three consecutive days to White Leghorn laying hens. The amount of individual compound dosed was approximately 7.5 mg per hen per day. Radiocarbon was eliminated very rapidly in excreta. Tissue residues were generally very low with the exception of fat depots, but trace amounts of radiocarbon residues were still detectable in fat, liver and kidney 11 days post-treatment. Residues were detected in the egg albumen of hens treated with cypermethrin and cis isomer of the acid, but none was detected from trans acid. Metabolites identified included products arising from the hydrolysis of the ester bond of cypermethrin, followed by oxidative attack on one or both methyl groups. The rate of oxidative attack on methyl groups was influenced by the geometry of the dichlorovinyl group. On the basis of the metabolites identified a metabolic pathway for cypermethrin in laying hens was proposed which included hydrolysis of the ester bond followed by oxidation of methyl group(s). The data indicate that use of cypermethrin as recommended should not produce residues in eggs and meat of laying hens at levels that are of toxic concern.