We have previously reported that plasma factor VIII levels are elevated in hyperthyroidism and present here the effects of oral thyroid hormone and oral beta-adrenergic blockade on factor VIII in normal male volunteers. Oral levothyroxine in 14 volunteers and oral liothyronine in nine volunteers produced significant percentage increases in resting pulse and factor VIII-related activities (factor VIII coagulant activity, factor VIII-related antigen, and ristocetin co-factor activity). The addition of oral beta-adrenergic blockade significantly reduced or prevented elevations in resting pulse but not increases in factor VIII-related properties. These findings are discussed with reference to possible regulatory mechanisms for plasma factor VIII levels related to alterations in thyroid function.