A possible interaction between dl propranolol and hyperparathyroid plasma ultrafiltrate on guinea pig auricles was studied in vitro. Plasma ultrafiltrates were sampled in 2 patients on chronic hemodialysis before (pre-PTx) and after (post-PTx) parathyroidectomy. A significant inhibition of propranolol depressant activity on cardiac contractile strength was observed in the presence of pre-PTx plasma ultrafiltrates. No such inhibition was noted in the presence of post-PTx plasma ultrafiltrates. An interaction is suggested between PTH and .beta. adrenergic blocking drugs at the cardiac level. Such an interaction could have important clinical implications.