Abstract: Studies were carried out to examine the role of phosphodiesterase(PDE) III and PDE IV in the development of tachyphylaxis of isolated guinea pig trachea to β₂ adrenoceptor agonists. Treating trachea with salbutamol(SB) 1 μmol·L ^-1 for 1h produced a 5-fold rightward shift of the SB concentration response curve for the spasmolytic effect against methacholine induced bronchocon traction and decreased the maximum SB induced relaxation by 30%, i.e. induced tolerance of airway response of SB in vitro . The PDE IV inhibitor rolipram (Rol, 1 μmol·L^-1, IC₅₀), but not the PDE III inhibitor siguazodan (SK&F 94836, 1 μmol·L^-1, IC₅₀), reversed the SB tolerance. However, the protein synthesis inhibitor cycloheximide(10 μmol·L^-1) did not abolish the SB tolerance. These results indicate that the SB tolerance may be related to increase in PDE IV activity.