Because of improved patient tolerance and decreased risks of idiosyncratic reaction, low-osmolarity contrast media are increasingly used for excretory urography. However, physiologic differences among patients may affect the optimal time for acquiring diagnostic radiographs during the pyelographic phase of the examination. A prospective, randomized, physician-blinded comparative study of 60 adult patients was undertaken to determine if the time to peak opacification of the pyelocaliceal systems differs with different doses of nonionic and ionic contrast media. Three doses of contrast media were used: a high dose (based on body weight) of a high-osmolarity ionic contrast medium, a high dose (based on body weight) of a low-osmolarity non-ionic contrast medium and a lower, fixed dose of a low-osmolarity nonionic contrast medium. The diagnostic quality of the radiographs did not differ statistically with the dose or the contrast medium. However, acquiring an additional radiograph during abdominal compression greatly increased the chance of obtaining at least one radiograph with maximal diagnostic information during the pyelographic phase. Despite potential differences among contrast media in the degree of pyelocaliceal opacification or distension and diuresis, it is not necessary to modify the timing of film acquisitions during excretory urography when lower doses of low-osmolarity agents are administered.