AbstractFrom a homogenate of rabbit colon a crude protein kinase was isolated. The phosphorylation of histone by this enzyme was maximal at 20 mM NaF. Higher concentrations inhibited the incorporation of32P. After chromatography on a DEAE‐cellulose column the crude protein kinase was partly purified. Cyclic AMP and cGMP stimulated the protein kinases eluted at 0.09 M‐NaCl (Type I) and at ∼0.2M‐NaCl (Type II). The main peak was of type II. Mix (l‐methyl‐3‐isobutylxanthine) relaxed the rabbit colon muscle. The increase in the protein kinase activity was closely correlated to the changes of the cAMP level. The cGMP content was also increased. Isoprenaline increased the cAMP level and the protein kinase activity of the colon muscle. Isoprenaline had no effect on the cGMP level. It is suggested that relaxing drugs which increase the cAMP level of rabbit colon muscle may activate protein kinase. This enzyme might be an important factor of the cAMP mediated relax