SUMMARY Three parameters of the diuretic response to a water load are defined these are: diuretic lag, rate of diuresis and total output/input. A heterogeneous stock of mice was set up using the strains A/Cam, CBA/FaCam, SF/Cam and Peru. Directional selection was made from this stock for rate of diuresis, and total output/input. The initial generations of selection showed positive and significant parent-offspring regressions and hence demonstrated genetic variation in these parameters. Two positive disruptive selection experiments were set up to explore the relationship between diuretic lag at 4 and 8 weeks of age. These experiments show also that there is genetic variation in diuretic lag and that gene manifestation at 4 weeks and at 8 weeks is different. The genetic variation affecting diuretic response to water load is discussed with respect to concepts of 'normal' physiology.