Free flow microperfusion (closed feedback loop) experiments in halothane-nitrous oxide-anaesthetized rats demonstrate two distinct oscillating TGF pressure responses to pertubations in early distal tubular flow. Flow rate changes of 2-8 nl min-1 induced a 0.02-0.03 Hz rhythm, while changes above 25 nl min-1 often induced a 0.17-0.18 Hz rhythm in proximal tubular pressures. Following interruption of microperfusion, the intratubular pressure showed an initial undershoot and a gradual oscillating return to the control level. Each nephron responded individually to systemic changes with oscillations differing in frequency, phase and amplitude from that of its neighbour. Postglomerular vascular pressure (PSV) oscillated synchronous with the proximal luminal pressure (PProx) of the same nephron; the amplitude of the PSV might be larger than that of the corresponding PProx. The PSV decreased by 2-3 mmHg as Henle loop flow was increased by 7 nl min-1. Furosemide (0.1-0.2 mM) in the microperfusate abolished the oscillations, and caused PSV and PProx to increase by 3-4 mmHg. This effect was rapidly reversible.