A double-blind crossover trial was carried out in 7 healthy male volunteers to investigate the effects of penbutolol and a placebo on plasma atrial natriuretic peptide (ANP) and antidiuretic hormone (ADH) levels before and after exercise. Each subject underwent several bicycle ergometric exercises lasting 6 min before and after the application of test medications. Ergometric exercises were performed before medication, and at 2, 5, 9 and 24 hours after medication. Blood samples for ANP and ADH levels were drawn before, after 15 min, after 2 hours (immediately after ergometry) and 5, 7, 9, and 24 hours after medication (immediately before ergometry). Urine was collected as follows: -2 to 0, 0 to 2, 2 to 4, 4 to 7, 7 to 14 and 14 to 24 hours after medication, and the volume as well as sodium excretion were documented. Penbutolol caused suppression of the exercise-induced increase in ANP. The 2 to 4 hour fractional sodium excretion was significantly decreased from 12.1 +/- 4.9 mmol/fraction after placebo treatment to 7.8 +/- 3.0 mmol/fraction after penbutolol application (p less than 0.03). There were no differences in the urinary outputs between penbutolol and placebo until 4 hours after medication, but penbutolol caused the total urinary output to increase from 1390 +/- 388 ml/24 hr during placebo treatment to 1725 +/- 549 ml/24 hr (p less than 0.02) due to the last collection fraction. Blood pressure and pulse rate both decreased during exercise after penbutolol. As opposed to the suppressing influence of penbutolol on ANP, ADH plasma levels were increased after exercise.(ABSTRACT TRUNCATED AT 250 WORDS)