alpha 1-Adrenergic agonists are known to be involved in the regulation of hepatocyte proliferation after a partial hepatectomy. The blockade of alpha 1-adrenergic receptors with the specific antagonist prazosin inhibits DNA synthesis which peaks at 24 h after surgery. In this report we have studied the effects of prazosin administration on several events occurring during liver regeneration. The results show that the nuclear volume and nucleolar volume density of hepatocytes were increased and that the relative amount of heterochromatin decreased at 24 h. The increase in hepatocyte nucleolar volume density and the decrease in the relative amount of heterochromatin were partially abolished by prazosin administration while the increase in the nuclear volume was not affected. The relative amount of the histone H1 variant H1(0) was reduced in 24-h regenerating liver and prazosin treatment prevented this reduction.