The IgE-mediated hypersensitivity to Ascaris antigen in reactor rhesus primates was used to assess the pharmacologic profile of U-66,858 (1-acetoxy-2-n-butyl-4-methoxy-naphthalene). When the compound was given by the oral route, it showed dose-related inhibition of resistance (RL) and compliance (Cdyn) changes. When the compound was given by the aerosol route, it showed dose independent inhibition. In 15 animals, aerosols (52 ± 32 to 53 ± 10% for RL, p = 0.05 and 45 ± 19 to 28 ± 19% Cdyn inhibitions, p = 0.05) for 5.0–0.1% aerosol. By the oral route, inhibition was seen at 1–4 h following administration. In 5 animals, oral doses of 10 and 5 mg/kg inhibited (RL by 98 ± 2 to 78 ± 1.5 %, p = 0.01 and Cdyn by 75 ± 17 to 60.9 ± 9.1 %, p = 0.05) by 10 and 5 mg/kg U-66,858, respectively. The in vivo demonstration of inhibition of pulmonary bronchoconstriction by this compound, in a model known to be leukotriene sensitive, coupled with its potent in vitro inhibition of 5-lipoxygenase enzymes, suggests this compound may be of use in 5-lypoxygenase-mediated models of asthma.