Patterns of cAMP-dependent protein kinases and cAMP-independent histone and casein kinases of hyperplastic rat thyroid glands and of human nodular non-toxic goitres have been analysed in two subcellular compartments, cytosol and particulate fraction. In hyperplastic rat glands the different compartmentalization of the two cAMP-dependent isoenzymes was preserved and the pattern of cAMP-independent protein kinases was not changed qualitatively, but the activities of the three classes of protein kinases were enhanced to different degrees: the highest increase was observed for the cAMP-dependent enzymes and the lowest for the cAMP-independent casein kinases. Analysis of individual peaks of cAMP-dependent kinases showed selective stimulation of the cytosolic Type II form and independent control of the Type I activity in the two subcellular compartments. Among cAMP-independent protein kinases only two histone kinase peaks were selectively enhanced; other kinase entities were changed to a lesser degree. During the involution of hyperplastic glands, a transient but differential enhancement of nearly all kinases was noted at first, which was followed later by a strong decrease, more or less rapid, of almost all kinase activities. In the regressed glands, neither the thyroid weight nor the pattern of different kinase entities corresponded to those of control, untreated glands, indicating that some of the protein kinase alterations in hyperplastic tissues are partly irreversible. In spite of great similarities, human and rat goitre tissues are not identical. The most important difference was the loss of compartmentalization of the cAMP-dependent isoenzymes in human tissue. The different ratios of the light and heavy peaks of cytosolic cAMP-independent histone kinases was the second characteristic which distinguished human and rat glands.