In 12 to 18-week-old Wistar-Kyoto rats, regular oscillations in the proximal intratubular pressure occurred spontaneously. The median frequency was 29.7 mHz (range 20-46.7 mHz). In spontaneously hypertensive rats, spontaneous oscillations also occurred, but these were highly irregular. In both strains, oscillations could be elicited by free flow microperfusion with artificial tubular fluid. When furosemide was added to the artificial tubular fluid in a concentration of 0.1 mmol/l, the oscillations were abolished in both strains of rats. It is concluded that, in both strains of rats, the oscillatory phenomena depend upon tubuloglomerular feedback activity, and that the differences in the oscillatory patterns between the two strain of rats represent a difference in the parameter setting of the tubuloglomerular feedback system.