1. The effect of an 18, 30 or 40 h fast on the rate of purine biosynthesis de novo as measured by the incorporation of [14C]formate into formylglycinamide ribonucleotide during azaserine block has been examined in the rat spleen.
2. There is a reproducible fall in the rate of purine biosynthesis de novo during fasting. Values at 40 h were approximately one-third of the values in the colony-fed non-fasted animals.
3. Re-feeding animals with an amino acid mixture by stomach tube partially reversed the inhibition of purine synthesis induced by a 40 h fast.