We studied renal opacification during intravenous urography in 20 rats subjected to glycerol-induced acute renal failure, with and without prior saline loading. Animals drinking saline suffered less severe renal impairment (mean plasma creatinine 175 mumol/l) than those drinking tap water (mean 341 mumol/l). Nephrograms occurred during intravenous urography in 19 rats at all levels of renal impairment, but were only followed by pyelograms in 8 with mild plasma creatinine elevation (mean 84 mumol/l). The occurrence of a pyelogram was determined not by whether an animal was drinking saline or tap water, but only by the level of renal impairment. The presence of the nephrogram throughout the range of functional impairment in acute renal failure argues against glomerular filtration as the principal route by which contrast medium reaches the nephron to produce this clinically very useful sign.