Single shot glomerular filtration rate measurements involving chelates (113mIn-DTPA, 99mTc-DTPA etc) assume direct loss from the plasma to urine via glomerular filtration and excretion. Inherent errors, due to considerable uptake of activity in tissue and uncertainty of complete bladder emptying are ignored and taking of half-hourly blood and urine samples involves patient discomfort. This paper describes a simple method of measuring urinary clearance of chelates using serial external arm counting which entails only an initial injection and takes into account tissue loss from plasma. The resultant plotted curve exhibits three phases, the first two depicting input and equilibration between plasma and tissue and the third and exponential part, which is a measure of the biological half-life of the chelate, being representative of the efficiency of renal glomerular filtration, the parameter to be measured. Results obtained, compared with single shot glomerular filtration rate measurements performed simultaneously, gave better correlation with clinical data including renography.