De novo purine synthesis has been examined in two genetic lines of chickens selected for their plasma uric acid levels. Synthesis rates were determined using aqueous extracts of tissue acetone powders. The high uric acid line (HUA) had significantly greater kidney synthesis rates compared to the low uric acid (LUA) birds (P less than .05). The mean value of kidney de novo uric acid synthesis rates was 5.65 +/- .65 micrograms and 3.6 +/- .24 micrograms uric acid synthesized/mg acetone powder/hour, respectively, for the HUA and LUA birds. Kidney uric acid synthesis rates were significantly correlated to plasma uric acid levels (P = .07). No correlation was seen between liver de novo uric acid synthesis rates and plasma uric acid levels. Plasma uric acid levels were monitored in both lines from hatching through 42 weeks of age. In the HUA line peaks of uric acid level were noted during the 2nd, 4th, and 8th week followed by a fairly abrupt rise in levels after the 20th week, whereas in the LUA males a broad peak in the early weeks was followed by a gradual rise in levels after the 20th week. The LUA females had slightly elevated levels during the first weeks; however, no significant changes occurred after the 20th week. Xanthine dehydrogenase (XDH) activities in kidney acetone powders from the HUA line were significantly higher compared to those from the LUA line (P less than .05). The mean value of kidney XDH activity was 85.41 +/- 13.81 and 46.35 +/- 6.71 mmoles NAD reduced per milligram acetone powder per hour, respectively, for the HUA and LUA lines. There was no significant correlation between liver XDH activities and plasma uric acid levels; however, the activity was significantly higher in the HUA than in the LUA line.