Selective renal angiography causes a biphasic change in renal blood flow and vascular resistance. In this study, 5 ml of meglumine/sodium diatrizoate (288 mg I/ml, 1455 mosm/kg), metrizamide (290 mg I/ml, 593 mosm/kg), isontonic saline (287 mosm/kg) and hypertonic saline (1500 mosm/kg) were injected into the renal arteries of seven adult mongrel dogs to determine whether the minimum flow (or maximum resistance) was related to the osmolality of the injected agent. The maximum resistance response was significantly smaller for metrizamide (20 +/- 4%) and isotonic saline (19 +/- 2%) than for diatrizoate (36 +/- 6%) or hypertonic saline (50 +/- 7%). Hypertonic saline produced two distinct types of responses: the typical biphasic response or a severe immediate drop in flow. Thus the maximum resistance response was related to agent osmolality. The "injection artifact," or flow changes occurring during the injection, were different for the four agents, and these differences appeared correlated to agent viscosity. Although both contrast media caused relatively small changes in renal hemodynamics, metrizamide caused significantly smaller changes than meglumine/sodium diatrizoate.