beta-Adrenergic relaxation of smooth muscle by catecholamines has been associated with elevated levels of cyclic AMP. The question arises whether subsequent activation of cyclic AMP-dependent protein kinase has a role in the regulation of smooth muscle contraction. There is substantial evidence that a Ca2+-activated myosin light chain kinase/phosphatase system regulates smooth muscle contraction, and Adelstein et al. have shown that the catalytic subunit of cyclic AMP-dependent protein kinase plays a part in this regulation, by phosphorylation of the high molecular weight subunit of the light chain kinase, which results in a decrease in the activity of the kinase. Here we have shown for the first time that the catalytic subunit of the protein kinase inhibits Ca2+-activated tension in skinned smooth muscle fiber preparations.