Abstract
There was greater than 4-fold strain differences in the clearances of sodium, potassium, osmolality and free water among mice of 24 inbred strains and F1 hybrids. These differences were detectable in spite of high within-strain variations. Even under highly standardized conditions about 50% of the total variance was ascribed to the within-strain component. Males showed more significant strain differences than females, and there were more differences between strains for osmolar and free water clearances than for sodium or potassium. Most environmental correlations were not significant, while many significant genetic correlations were found between the clearances and bodyweight and, less strongly to kidney weight. Bodyweight was therefore selected as a common base, and the exponent was about 0·80 for both sodium and potassium clearance.