40 patients with MVP (mean age 36 years--group I) and 20 controls without any heart disease (mean age 40 years--group II) after thorough clinical examination (including Holter monitoring) were subjected to electrophysiological study for sinoatrial node function assessment. The following parameters were studied (before and after "pharmacological denervation"):, SNRT, CSNRT, SP and SACT (using direct method and Strauss and Narul's method). SN dysfunction was found in 19 patients with MVP (37.5%) and in 3 controls (15%) in 24-hour Holter ecg. In the electrophysiological study SN dysfunction was diagnosed in 13 patients (32.5%) of group I. 10 of them displayed also electrocardiographic symptoms of SN dysfunction. "Pharmacological denervation" of the heart (propranolol 0.1 mg/kg, atropine 0.02 mg/kg) revealed 4 cases of concealed SN dysfunction defining the functional background of abnormalities in 7 out of 13 patients. Our data show that SN dysfunction is common in patients with MVP. Electrophysiological study performed with "pharmacological denervation" may disclose cases of concealed SN dysfunction and define them as functional or organic.