This study was designed to investigate the decreased urinary kallikrein excretion (Ukall.V) in Okamoto-Aoki spontaneously hypertensive rats (SHR) and the effect of long-term converting enzyme inhibition. From ages 4 to 7 weeks, Ukall.V was determined (amidolytic assay: nanokatals/wk) in 4 groups of 6 male rats housed into individual metabolic cages and fed a normal sodium diet: SHR and normotensive Wistar-Kyoto rats (WKY); SHR-C and WKY-C which were given captopril: 30 mg/kg BW every 12 hours by gavage. Ukall.V was each time lower in SHR than in age-matched WKY, even at 4 wks of age (54.6 +/- 9.1 vs 108.5 +/- 16.1 nkat/wk; p less than .01) when systolic blood pressure (s.BP) was already higher. In SHR-C, s.BP was identical or slightly lower to that in WKY. Ukall.V was still lower at wk 4 when captopril was first administered (60.9 +/- 8.4 nkat/wk; p less than .01), but identical to that in WKY at each subsequent age (105.7 +/- 25.9 vs 114.1 +/- 5.6 nkat/wk at wk 5; 219.8 +/- 44.5 vs 253.4 +/- 22.4 nkat/wk at wk 7). Excretion of active kallikrein was highly correlated to s.BP in WKY (r = .87), SHR (r = 0.91) and SHR-C (r = 0.95). The slope of the regression line relating Ukall.V with s.BP was significantly less in SHR than WKY (1.33 +/- 0.35 vs 3.36 +/- 0.84 nkat/wk/mmHg; p less than .01); the slope in SHR-C (3.35 +/- 0.77 nkat/wk/mmHg) was significantly steeper than in SHR (p less than .01) and identical to that in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)