Cyclic AMP-dependent and -independent protein kinase have been isolated from livers of rats at 3, 9, 18 and 30 months of age. A preliminary characterization reported here reveals that in addition to a decrease in their activities due to ageing and a likely shift to cyclic AMP-independent casein kinase activity at 30 months of age, the enzymes show modest changes in some of their kinetic properties. In the case of cyclic AMP-dependent enzymes, decreases in Vmax for ATP and Km for cyclic AMP as well as histones were observed. On the contrary, casein kinases displayed increases in Km values for casein, GTP and ATP. Molecular weights of 143,000 daltons for casein kinases from 3-, 9- and 18-month-old rats and 211,000 daltons for that from 30-month-old rats were obtained. Slight differences were also noticed in the inhibitor sensitivity (NaF and 2,3-diphosphoglyceric acid) of casein kinases isolated from rats of different ages.