We investigated the renin-angiotensin system in the genetically hypertensive (HBP), normotensive (NBP) and low blood pressure (LBP) mice developed by G. Schlager, one of the authors. Renin in the plasma, kidney and submaxillary gland was determined by enzymatic assay and by direct radioimmunoassay (DRIA). Trypsinization of mouse plasma was also investigated. PRA and plasma renin content were not significantly different in the different lines, sexes and generations. Trypsinization of the plasma revealed the presence of inactive renin, as has also been found in humans, hogs, dogs and rats. The proportion of active renin against trypsinized total renin was about 54-77% and was not significant in the different lines, sexes and generations. There was also no significant difference in renal renin content in the various lines, sexes and generations. However, in the submaxillary gland, renin content and activity were high in male mice, in every line. These data suggest that the renin-angiotensin system may not contribute to the established phase of blood pressure.