Two contractile proteins, troponin-I and C-protein, are rapidly phosphorylated in perfused heart in response to stimulation by catecholamines. Both are phosphorylated only in response to increases in intracellular cyclic AMP, and are good substrates for cyclic AMP-dependent protein kinase in vitro. The phosphorylation of troponin-I decreases the affinity of the troponin complex for Ca2+ by increasing the off-rate for Ca2+ dissociation. This is probably part of the mechanism whereby catecholamines increase the rate of relaxation in heart. C-protein is phosphorylated in vivo and in vitro on 4–5 sites, but the function of both C-protein and its phosphorylation are unknown. Myosin P-light chain (phosphorylated by a Ca2+-dependent kinase) is approximately 50 % phosphorylated in control perfusions, and this is unchanged during acute inotropic interventions. However, perfusions with 32Pi indicate an active kinase/phosphatase couple, which may be fully active under control conditions.