Metabolism of juvenile hormone (JH) in 11-day-old D. virilis flies from resistant and sensitive lines and in hybrids of these lines was analyzed after short-term heat stress (4 h, 38 degrees C). The level of JH hydrolysis after stress remained constant in the sensitive line, but drastically decreased in the resistant one. Between-line differences of the metabolic JH response to stress were shown to be controlled monogenically or by a block of closely linked genes. The allele of the resistant line was dominant.