The determinants of glomerular ultrafiltration in superficial glomeruli of a strain of English cross-breed rabbits have been studied using micropuncture techniques. Mean arterial blood pressure in the anaesthetised rabbits was 70 +/- 2 mmHg. The glomerular filtration rate in the kidney prepared for micropuncture was 4.4 +/- 0.4 ml/min, the filtration fraction was 22 +/- 1% and renal blood flow was 33 +/- 3 ml/min, and these values were comparable to values in conscious rabbits. Glomerular capillary pressure (Pgc) averaged 31 +/- 1 mmHg, the single-nephron glomerular filtration rate (SNGFR) averaged 25 +/- 2 nl/min, and the mean ultrafiltration pressure (calculated using the whole-kidney filtration fraction) averaged 7 +/- 1 mmHg. A net positive pressure at the efferent end of the glomerular capillaries (4.4 +/- 0.9 mmHg) indicated that a state of filtration pressure disequilibrium existed, under the experimental conditions of this study, in rabbit glomeruli. The calculated glomerular ultrafiltration coefficient (Kf) was 0.08 +/- 0.01 nl s-1 mmHg-1. Thus, compared to the Munich-Wistar rat, SNGFR is lower in the rabbit. This reflects the substantially lower glomerular ultrafiltration pressure in the rabbit, although this was offset partially by a higher Kf.