SUMMARYThe cellular mechanisms underlying smooth muscle hypoxic relaxation were examined in experiments on isolated chemically skinned muscle strips of rat portal vein. Low Po2 (3 9 kPa)shifted the pCa-tension relation to the right by 0-29 + 001 pCa units as compared to control curves (PO2,. Thus the Ca2+ sensitivity of the filaments had decreased. Addition of cyclic AMP (30 ,umol 1-1) with theophylline (5 mmol 1-1) to the buffer solution produced a similar shift to low Po2. Low Po, also decreased the maximal force to 40-50% of control.Inclusion of sodium fluoride (10 mmol 1-1) and aluminium chloride (10,umol 1-1) in a bath solution caused partial (25-30%) relaxation of skinned smooth muscle preconstricted with 1O #smol 1-Ca2+. The rate and amplitude of smooth muscle relaxation at low Po were significantly decreased by tolbutamide (5 mmol 1-1), which is known to inhibit cyclic AMPdependent protein kinases. We suggest that Po2 changes can alter myofilament responsiveness to Ca2`and this effect may be related to cyclic AMP-dependent phosphorylation of myosin light chain kinase, its inactivation and subsequent uncoupling between Ca2+ and contractile machinery in smooth muscle.