Glomerular filtration rate (GFR) is an important index of renal function and routinely used in patient care and basic research to evaluate progression of renal diseases or test the efficacy of novel therapeutic strategies. Determination of GFR in mouse models has been mostly practiced in anesthetized animals, which is not suitable for serial monitoring of GFR in the individual mouse. In this chapter, we outline two approaches for determining GFR in conscious mice including 1) determination of urinary excretion of fluorescein-labelled inulin (FITC-inulin), and 2) determination of plasma FITC-inulin decay following a single bolus injection. The GFR values determined using these two methods are comparable. The sensitivity of the methods in reflecting renal function was validated in nephrectomized mice and early stage diabetic mice. The effects of inbred mouse genetic background on GFR values are also discussed in this chapter.