The microbial protein supply to ruminants can be estimated based on the amount of purine derivatives (PD) excreted in the urine. Four experiments were conducted to evaluate the PD excretion method for Bali and Ongole cattle. In the first experiment, six male, two year old Bali cattle (Bos sondaicus) and six Ongole cattle (Bos indicus) of similar sex and age, were used to quantify the endogenous contribution to total PD excretion in the urine. In the second experiment, four cattle from each breed were used to examine the response of PD excretion to feed intake. {sup 14}C-uric acid was injected in one single dose to define the partitioning ratio of renal:non-renal losses of plasma PD. The third experiment was conducted to examine the ratio of purine N:total N in mixed rumen microbial population. The fourth experiment measured the enzyme activities of blood, liver and intestinal tissues concerned with PD metabolism. The results of the first experiment showed that endogenous PD excretion was 145 {+-} 42.0 and 132 {+-} 20.0 {mu}mol/kg W{sup 0.75}/d, for Bali and Ongole cattle, respectively. The second experiment indicated that the proportion of plasma PD excreted in the urine of Bali and Ongole cattle was 0.78 and 0.77 respectively. Hence, the prediction of purine absorbed based on PD excretion can be stated as Y = 0.78 X + 0.145 W{sup 0.75} and Y = 0.77 X + 0.132 W{sup 0.75} for Bali and Ongole cattle, respectively. The third experiment showed that there were no differences in the ratio of purine N:total N in mixed rumen microbes of Bali and Ongole cattle (17% vs 18%). The last experiment, showed that intestinal xanthine oxidase activity of Bali cattle was lower than that of Ongole cattle (0.001 vs 0.015 {mu}mol uric acid produced/min/g tissue) but xanthine oxidase activity in the blood and liver of Bali cattle was higher than that of Ongole cattle (3.48 vs 1.34 {mu}mol/min/L plasma and 0.191 vs 0.131 {mu}mol/min/g liver tissue). Thus, there was no difference in PD excretion between these two breeds. Liver uricase in Bali and Ongole cattle was 1.46 and 1.17 {mu}mol/min/g tissue, where as no activity was detected in the intestinal tissue and blood. (author) 9 refs, 7 figs, 9 tabs