1. Spontaneously hypertensive rats (SHR) excrete less kallikrein in urine than Wistar-Kyoto rats (WKY) during the developmental phase of hypertension. The present study was designed to examine whether the urinary defect is related to abnormalities in the renal kallikrein content in this hypertensive model. 2. Active and total kallikrein were measured (amidolytic assay) in the renal cortex of newborn and 4-, 8- and 12-week-old SHR and age-matched WKY. Active and total kallikrein were also measured in urine at the same ages, except at birth. 3. Tissue active kallikrein was significantly lower in SHR at birth, representing on average 53% of the values in WKY expressed as content per total cortex weight. Tissue total kallikrein did not differ between newborn SHR and WKY. 4. SHR at 4, 8 and 12 weeks of age had lower urinary active and total kallikrein excretion. Tissue active akllikrein, but not total kallikrein, was higher than in age-matched WKY per g of cortex weight or per mg of protein, whereas both tissue active and total kallikrein were lower in SHR when expressed as content per total cortex weight. At these three ages, active kallikrein represented, on average 86%, while total kallikrein represented 77%, of the values in age-matched WKY. 5. Our results indicate a defect in prokallikrein activation rather than in kallikrein synthesis in the renal cortex of SHR at birth. The reduction in urinary kallikrein excretion during the developmental phase of hypertension in young SHR is similar to the reduction observed in the renal tissue.