Urinary kallikrein excretion was studied in rats bred for susceptibility and resistance to the hypertensive effect of salt. The rats were on a regular rat chow diet (0.45% sodium content) and tap water and were not hypertensive at the time of the study. Urinary kallikrein excretion, measured by kinin radioimmunoassay, was 10 to 20 times lower in the susceptible rats than in the resistant rats (4.39 +/- 1.61 microgram/24 hours and 56.4 +/- 5.8 microgram/24 hours, respectively; P less than 0.001). Urinary kallikrein excretion was also measured in New Zealand genetically hypertensive rats and in normotensive Wistar-Otago rats (controls). Kallikrein was found to be significantly lower in the genetically hypertensive rats than in the controls (49.1 +/- 6.2 microgram/24 hours and 76.8 +/- 6.9 microgram/24 hours, respectively); however, when expressed per 100 g of body weight, there was no significant difference. In conclusion, although urinary kallikrein excretion per rat was decreased in the genetically hypertensive rats when compared with the controls, this difference could be caused by the lower body weight of the genetically hypertensive rats. Urinary kallikrein excretion, when expressed per 100 g of body weight, is significantly lower in susceptible than in resistant rats. This could be a consequence of a genetic defect that may play a role in the development of hypertension, perhaps through alteration of renal function.