There is broad species variation in the type of cAMP-dependent protein kinase isozyme present in supernatant fractions of heart homogenates as determined by DEAE-cellulose chromatography, Isozyme I, which elutes at less than 0.1 M NaCl, is predominant in mouse and rat hearts; while isozyme II, which elutes at greater than 0.1 M NaCl, is the predominant type in beef and guinea pig. Human and rabbit hearts contain about equal amounts of the two types. The type I heart kinases are more easily dissociated into free regulatory and catalytic subunits by incubation with histone than are the type II kinases, and the separated regulatory and catalytic subunits of isozyme II of rat heart reassociate more rapidly than the subunits of isozyme I under the conditions used. The data from several experiments using rat heart indicate that the basal activity ratio of the protein kinase in crude extracts (approximately 0.15) is due mainly to basal endogenous cAMP and that cAMP elevation accounts entirely for the epinephrine effect on the enzyme. Addition of epinephrine and 1-methyl-3-isobutylxanthine to the perfusate causes a rapid (1 min) increase in cAMP, active supernatant protein kinase, and active phosphorylase in perfused hearts of both rat (mainly isozyme I) and guinea pig (mainly isozyme II). The elevation percentage in cAMP is about the same in the two species, but the increase in active protein kinase is greater in rat heart. If hearts from either animal are perfused continually (10 min) with epinephrine (0.8 muM) and 1-methyl-3-isobutylxanthine (10 muM), the cAMP level, active protein kinase, and active phosphorylase remain elevated. Likewise, all parameters return rapidly to the basal levels when epinephrine and 1-methyl-3-isobutylxanthin are removed. Most of the epinephrine effect on the rat heart supernatant kinase is retained at 0 degrees if cAMP is removed by Sephadex G-25 chromatography, although this procedure completely reverses the epinephrine effect in the guinea pig heart. The epinephrine effect on the rabbit heart kinase (approximately equal amounts of isozymes I and II) is partially reversed by Sephadex G-25. These species differences can be accounted for by differences in association-dissociation behavior of the isozymes in vitro. The data suggest that epinephrine causes activation of both isozymes. The activity present in the particulate fraction comprises nearly half of the total cAMP-dependent protein kinase activity in homogenates of rabbit heart. Triton X-100 extracts of low speed particulate fractions from hearts of each species tested, including rat heart, contain predominantly or entirely the type II isozyme, suggesting differences in intracellular distribution of the isozymes. The binding of the protein kinase to the particulate fraction is apparently due to the properties of the regulatory subunit component. Differences in topographical distribution of the isozymes could provide for differences in either physiological regulation or substrate specificity.