We evaluated the in vivo and in vitro airway responses to cholinergic stimulation in two highly inbred strains of rats. Five Fisher and six Lewis rats inhaled aerosols of doubling concentrations of methacholine (MCh). Pulmonary resistance (RL) was measured before and after each MCh inhalation. The concentration of MCh required to double RL (EC200RL MCh) was calculated. Spirally cut tracheal strips were suspended in an organ bath and the isometric tension was recorded. Cumulative concentration-response curves to carbachol were obtained using concentrations ranging from 10(-9) to 10(-3) M in half-log increments. The maximum tension (Tmax) developed and the negative log of the molar concentration required to induce 50% of Tmax (pD2) were calculated. The geometric mean of ED200RL MCh for the Fisher group was significantly lower than that for the Lewis group (0.68 and 2.66 mg/ml, respectively; p less than 0.05). No significant difference was found for Tmax (0.67 +/- 0.11 vs. 0.50 +/- 0.11 g) and pD2 value (6.24 +/- 0.08 vs. 6.21 +/- 0.22 -log M), indicating a similar contraction of tracheal smooth muscle. We therefore conclude that (1) in vivo airway responsiveness is strain-related and genetically determined in the rat; (2) that there is no correlation between in vivo and in vitro airway response to cholinergic stimulation and (3) that intrinsic properties of airway smooth muscle do not account for the differences observed in airway responsiveness in vivo.