Cytosolic cAMP-dependent, cAMP-independent protein kinase (PK) and DNA-associated nuclear kinase activity were compared in the proliferating liver of rats: (1) fed with commercially available basal laboratory diet; (2) fasted (2 days) and then fed with basal laboratory diet (2 days); (3) fasted (2 days), supplied with 30% glucose (1 day), and then realimented with basal laboratory diet; (4) fasted (2 days), supplied with 30% glucose (1 day), and then given a single dose of 3,5,3 -tri-iodothyronine (T 3 ) with realimentation (1 day with basal laboratory diet). The incorporation of 1 4 C-thymidine into rat liver DNA was increased in animals supplemented with glucose (P < 0.01) and even more pronounced in these with (T 3 ) injection. Fasting significantly decreased both cytosolic cAMP-dependent and cAMP-independent PK activities in the rat liver, but not DNA-associated kinase activity, when compared with the ad libitum fed controls. Both of the cytosolic kinase activities were restored by glucose, while nuclear DNA-associated kinase activity was not altered. T 3 did not change the cytosolic kinase activities but significantly increased the DNA-associated kinase activity. The data obtained by Linewiever-Burk analysis of the cAMP-dependent protein kinase activity suggest that the lower activity in liver of fasted rats restored by glucose was not due to the induction of the enzyme. The specific binding of T 3 to rat liver nuclear receptors was characterized; the equilibrium association constant (Ka) and the maximal binding capacity (MBC) were evaluated. Fasting for 48 hr decreased MBC (P < 0.05), glucose administration with refeeding reversed this value to control levels. The data representing Ka were of the same value in all groups tested. These observations suggest that T 3 may play a role as a modulator of liver cell proliferation, potentially via nuclear DNA-associated kinase activity. cAMP-dependent and cAMP-independent cytosolic kinase activities may be necessary for permitting DNA synthesis stimulated by nutritional manipulations, but these cytosolic kinase activities are clearly not sufficient for stimulating elevated DNA synthesis: increased DNA-associated kinase activity appears to be critical. Our results also support the idea that the mechanism of hepatocyte proliferation in rats caused by a nutritional regimen may differ from that found in the liver regenerating after partial hepatectomy.