The study evaluated the potential of the plasma concentration of purine derivatives (PD) as an alternative to total urine collection method to predict the microbial nitrogen (MN) supply in crossbred bulls. In a completely randomized design, 15 growing crossbred bulls were divided into three groups and assigned three levels (100, 80 and 60%) of voluntary dry matter intake (VDMI) on a diet of wheat straw and concentrate mixture (50:50). A metabolism trial of ten days duration was conducted after 90 days of experimental feeding. The daily urinary excretion of allantoin and PD decreased significantly (P<0.01) with the reduction in feed intake while creatinine excretion remained similar in animals fed at different levels. The MN supply calculated from the PD excreted in total urine (40.16 to 68.08 g/d) was higher (P<0.01) at increased levels of feed intake. The concentration of PD in plasma reflected the differences in MN supply among animals fed at 100 and 80% of VDMI, while it could not predict the differences in same among animals fed at 80 and 60% of VDMI. Daily glomerular filtration rate (GFR) was found to be similar in animals fed at 100 and 80% of VDMI, while there was a fall in GFR, when feed intake was lowered to 60% of VDMI. The results of this study revealed that, if the plasma concentration of PD is to be used as an index of microbial protein supply, the variation in GFR needs to be considered. Therefore, PD measurement in total urine is a better indicator of MN supply than the plasma PD concentration.