In rats of the Milan hypertensive strain (MHS) the disease can be transplanted with the kidney to rats of the Milan normotensive strain (MNS). It has been found that GFR, salt and volume regulation differ between MHS and MNS rats. Tubuloglomerular feedback control mechanism (TGF) is important for body volume regulation and we therefore wanted to study the TGF control in MHS and MNS rats. Whole kidney and micropuncture experiments were done before and during saline volume expansion (5% of body weight). In an initial series of experiments measurements were made of total kidney GFR, urine excretion rate of sodium and potassium and subcapsular interstitial hydrostatic pressure (psc); interstitial oncotic pressure (pi int) was estimated from hilar lymph protein concentration. In a second series, proximal tubular stop-flow pressure (psf) was determined upstream from a wax block while the distal nephron was being perfused with Ringer solution at a flow rate varying from 0 to 40 nl X min-1. In this way the maximal drop in stop-flow pressure (delta psf) and also the turning point (TP), the tubular flow rate at which 50% of this response was achieved, could be determined after saline volume expansion and after 2 h of complete ureteral occlusion. The results showed that GFR was similar in MHS and MNS rats in the control situation, but that during volume expansion it was significantly lower in the MHS group. Interstitial psc and pi int and net interstitial pressure (psc - pi int) were similar in MHS and MNS rats both under control conditions and during volume expansion.(ABSTRACT TRUNCATED AT 250 WORDS)