Hemodynamic stability is better preserved during bicarbonate hemodialysis compared to acetate. We have studied the effects of bicarbonate (HDB) and acetate hemodialysis (HDA) on plasma levels of vasoactive substances. The treatments were performed for 270 min. A cuprophan plate dialyzer was used. The ultrafiltration volume and the ultrafiltration rate were identical in the individual patients during the two treatments. In the case of vasoconstrictors there was an increase in neuropeptide Y (NPY) (20%, p < 0.01) during HDB and arginine vasopressin (AVP) was unchanged. Unlike this was the response during HDA when there was no change in NPY and a decrease in AVP (38%, p < 0.01). An increase in noradrenaline (NA) (41%, p < 0.05) occurred during HDA different from what was the case during HDB. There was a gradual increase in renin (PRA) during both HDB (141%, p < 0.05) and HDA (148%, p < 0.01). With respect to vasodilators there were no differences between the two regimes regarding calcitonin gene-related peptide (CGRP) and motilin (MOT). The change in substance P (SP) during the treatments was also similar but somewhat more pronounced during HDB. Thus, an initial rise occurred (HDB, 81%, p < 0.01; HDA, 36%, p < 0.05) followed by a decrease (HDB, 26%, p < 0.05) or a tendency to decrease (HDA, 12%, p = 0.058) during the remaining part of the treatment. A rise in beta-endorphin (beta-END) occurred during HDB (10%, p < 0.05) but not during HDA. An increase in vasoactive intestinal peptide (VIP) occurred during HDB (27%, p < 0.05) different from the decrease during HDA (11%, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)