Methotrexate (MTX) is a potent anti-vitellogenic agent in the parasitic wasp Bracon hebetor Say (Hymenoptera: Braconidae). The disposition of MTX has been examined in B. hebetor by tracing the movement of super(3)H-MTX. About 30% of an oral dose moved from the gut into the physiological interior of the insect. The amount of MTX in the gut rapidly decreased with a corresponding increase in the level of drug inside the body. A small amount of MTX may be accumulated in the ovaries. MTX is rapidly eliminated from the body in an exponential manner with a half-life of 10.6 h. These results confirm that MTX exerts its anti-vitellogenic effect at the time of treatment, in that when the maximum toxic effects are seen, there is little MTX left in the body. Biochemistry; Metabolic biochemistry; Nutrition; Digestion; Abiotic factors; Chemical factors Bracon hebetor (Braconidae); Pollutant metabolism; Absorption; Organic chemicals; Tritium labelled methotrexate, metabolism & elimination