lodinated X-ray contrast agents and gadolinium chelate magnetic resonance imaging contrast enhancing agents, have the same pharmacokinetics as99mTc or51Cr labelled ethylenediaminetetraacetic acid and inulin. They are all excreted unchanged by passive glomerular filtration in the kidney. They may, therefore, be used to obtain measures of renal function as an adjunct to any imaging study involving contrast enhancement. The theory of this, and some applications, are outlined, and a warning is given concerning the possible effect on renal function of the agents themselves.