Urinary kallikrein excretion (UKE) was measured in 20 patients before and after hemodialysis (HD). When compared to the values of normal subjects (136 ± 56 μg/24 h, n = 100), UKE was decreased in all patients before HD (6.6 ± 5.8 μg/24 h, p < 0.001, n = 20). After HD a significant increase in 24-hour UKE was observed in all patients (18.6 ± 7.2 μg/24 h, p < 0.05, n = 20). Expressed as the individual percent increase, the UKE enhancement ranged from 16 to 670%. It was due to an enhancement in the excretion of the active form which represented 52 ± 6.8% before HD and reached 76.3 ± 7.5% of the total form after HD (p < 0.01). The excretion of the inactive form remained unchanged. The increase in UKE was found to be significantly correlated with reductions in urinary sodium, potassium, and osmolality (r =-0.826, r =-0.568, r =-0.847, respectively, p < 0.01, n = 20). The increase in UKE following HD could not be explain by an increase in aldosterone as urinary aldosterone decreased. A transient improvement in intracellular homeostasis (removal of inhibitory toxins and normalization of osmotic pressure) could be evoked. The increase in UKE 24 h after HD points out a new situation confirming the relations of UKE with changes in osmolality and extracellular volume.