With the use of micropuncture techniques, glomerular dynamics were characterized in euvolemic Munich-Wistar (Chapel Hil) rats. Single nephron glomerular filtration rate (SNGFR) and plasma flow (SNGPF) averaged 31 nl/min and 100 nl/min, respectively. Efferent effective ultrafiltration pressure (PUF) was greater than zero in each animal (7 mmHg, P less than 0.001), indicating filtration pressure disequilibrium. Mean PUF (PUF) was 15 mmHg and specific glomerular filtration coefficient (Kf) values averaged 0.033 nl/(s x mmHg). Similar experiments were performed on Munich-Wistar rats from B.M. Brenner's colony. Filtration pressure equilibrium was reached in rats with SNGPF < 112 nl/min; disequilibrium existed when SNGPF was > 168 nl/min. Overall, PUF (15 mmHg) was similar to that in Chapel Hill rats, SNGFR (49 nl/min) was greater, as was specific Kf (0.066 nl/(s.mmHg)). These observations document important Kf differences in the two colonies. In other experiments on Chapel Hill rats, plasma volume expansion produced increases in SNGPF and renal plasma flow (RPF). However, SNGFR (33 nl/min) and GFR (1.2 ml/(min x g kidney wt)) were unchanged, as were SNGFR/GFR and SNGPF/RPF ratios. SNGFR was relatively insensitive to increased plasma flow because of the relatively low Kf and filtration pressure disequilibrium during euvolemia. In additional experiments, GFR was also only weakly dependent on RPF in conscious rats.