The study evaluated the possibility of measuring the plasma concentration of purine derivatives (PD) as an alternative to urinary PD excretion to predict the microbial protein supply in goats fed different vegetable protein supplements. Four different iso-nitrogenous concentrate mixtures were prepared by using four different oil cakes for the feeding of animals in different groups (groundnut cake/GNC (Group I), mustard cake/MC (Group II), soybean meal/SBM (Group III) and cotton seed cake/CSC (Group IV)]. Eight adult Barbari goats were fed maintenance diets with wheat straw and either one of the concentrate mixture (50:50) in two sets of 4x4 Latin Square Design for a feeding period of 30 days. During the last 8 days of each feeding period animals were placed in metabolism crates for sample collection. Allantoin and total PD excretion (mmol/d) in urine was higher (P<0.05) in groups supplemented with GNC and SBM when compared to groups fed diets containing MC and CSC. The concentration (µmol/L) of allantoin and total PD in plasma was higher (P<0.05) in group III as compared to group II. However, the concentration (µmol/L) of uric acid, salvageable PD (xanthine and hypoxanthine) and creatinine in plasma was similar (P>0.05) among the dietary treatments. The plasma level of PD (Y, mmol/L) was poorly correlated (R2 = 0.44) with urinary PD (mmol/d) excretion. Therefore, from the present study it can be concluded that the plasma concentration of PD serves as a rough indicator of urinary PD excretion and further investigations are required to use it as a tool for precise calculation of rumen microbial nitrogen synthesis.