Hormonal control of glucose production and of L-pyruvate kinase activity has been measured in isolated liver cells from fed control and thyroidectomized rats. In hypothyroid rats, sensitivity to isoproterenol as measured by these parameters was increased: the apparent K0.5 for isoproterenol-induced stimulation of glucose production decreased from 8.0 +/- 3 X 10(-6) M in control rats to 2.0 +/- 0.2 X 10(-8) M in hypothyroid rats (P less than 0.001) and the apparent K0.5 for inhibition of L-pyruvate kinase was 5 +/- 2 X 10(-7) M vs. 7 +/- 2 X 10(-9) M (P less than 0.001) in control and thyroidectomized rats, respectively. Utilisation of specific adrenergic antagonists confirmed increased beta-adrenergic responsiveness in hypothyroid rats. This phenomenon was not reversed by 3 days of T3 treatment (10 micrograms/100 g body weight). Sensitivity to the alpha-agonist was unchanged by thyroid status. Stimulation of glucose production and inhibition of L-pyruvate kinase activity by glucagon and their reversal by insulin were not affected by hypothyroidism. The dose-response curve to vasopressin and its maximal effect measured on stimulation of glucose production were unchanged in thyroidectomized rats. Thus, hypothyroidism produces a specific enhancement of liver beta-adrenergic responsiveness without affecting sensitivity to glucagon, insulin and vasopressin.