SUMMARY The relationships between urinary kallikrein (Uk.,), and plasma renin activity (PRA), urinary aldosterone (U«ldo), Na+balance, SK+, and renal function were studied in essential hypertensives (EHT) and normals. Utalwas measured by a radiochemical esterolytic assay. We studied 18 white patients with EHT (15 men, 3 women) ages 31.6 ± 2.1 (SEM) yrs, BP 138 ± 2/95 ± 2 mm Hg. and 12 white normals (NLS) (7 men, 5 women) ages 30.2 ± 2 3 yrs, BP 112 ± 4/71 ± 2 mm Hg. All received a 5-day diet of 400 raEq Na+, 80 mEq K+/day, and 5 days of 10 mEq Na+, 80 mEq K+/day. All achieved Na+balance by Day 5. On Day 5 of the low Na+diet, 24 hr. U$, in EHT was 15.8 ± 2.4 (esterase units/24 hr) rs NLS, 17.0 ± 2.8. PRA was the same in EHT and NLS, but U.ldowas higher in NLS. (Day 5, low Na+, EHT, U«ld0− 29.4 ± 33 Mg/24h. vs NLS 41.8 ± 4.7, p< 0.02). Analysis of individuals showed that all NLS increased U after salt restriction, while 3 EHT decreased U$j after salt restriction. This abnormal response In EHT was not related to abnormalities in Undo, PRA, Na+balance, SK+, or creatinine clearance. In 3 EHT with low-renln EHT, toe UkaJresponse was normal. In two of four patients with primary aldosteronism, Ut., was normal despite increasedU.ido- The Urn response to salt restriction is abnormal in some EHT, unrelated to U$ao or PRA, suggesting either a primary defect in U$, and/or the presence of other factors modulating U$, in EHT.