The effects of various dietary fibers on the metabolism of the organochlorine insecticide, lindane, were compared. Groups of six weanling female Sprague-Dawley rats were fed either a synthetic low-residue diet (LRD), LRD + 10% pectin, LRD + 10% agar, LRD + 10% cellulose, or Purina Lab Chow for 28 days. The animals were then dosed orally with 2.87 mg lindane and were killed 24 hr later. A smaller proportion of administered radioactivity was recovered in the excreta and selected tissues from the rats fed the LRD diet than from other groups and the fate of the radioactivity not accounted for was investigated in a second experiment using rats fed either LRD unsupplemented, LRD + 10% pectin or the standard chow diet. Pectin and the dietary fiber contained in Purina Lab Chow caused significant alterations in the metabolism of lindane. A significant increase in the excretion of radiolabelled products, a higher level of conjugated chlorophenols and polar metabolites, a significant alteration in the proportions of the excreted chlorophenols and significant stimulation of the enzymes involved in lindane metabolism indicated that dietary fiber such as pectin or the plant fiber in Purina Lab Chow can significantly affect the metabolism of xenobiotics in mammals.