Smooth muscle from guinea pig taenia coli was chemically skinned with Triton X-100 and stored in ATP-salt solution containing 50% glycerol at -20 degrees C. Fiber bundles were relaxed at Ca2+-concentrations below 10(-7) M, but contracted at 10(-6) M Ca2+. The isometric tension developed could be partly relaxed by the addition of c-AMP (in the presence of NaF), and it could also be inhibited following preincubation with the catalytic subunit of c-AMP dependent protein kinase. The inhibitory effect was much more pronounced at intermediate Ca2+-concentrations (e.g. 10(-6)) than at concentrations producing a maximum contraction, suggesting that Ca-sensitivity had been lowered. Sodium fluoride which was required to potentiate the c-AMP effects was found to have a slight relaxing effect per se. The c-AMP effect may be mediated through activation of cyclic AMP-dependent kinase, producing, phosphorylation of the myosin light chain kinase which, according to Adelstein et al. (1978), may result in a net dephosphorylation of the myosin light chains and a concomittant inhibition of the contractile response.