1. In the yeast Schizosaccharomyces pombe 972h-, the uptake rate of both adenine and guanine is related to variations in the specific activity of the corresponding phosphoribosyltransferases during the growth of the culture. Furthermore, the mutant strains dap 1, devoid of adenine phosphoribosyltransferase activity, and pur 1, devoid of guanine phosphoribosyltransferase activity have a lowered uptake rate of adenine and guanine respectively, along with an increased apparent Km value for these purines in comparison to the wild-type 972h. 2. The uptake rate of the purines is strongly dependent on the pH of the uptake medium in 972h- as well as in the strains dap 1 and pur 1, the optimum being between pH4 and pH5. 3. A new method of extraction of 5-phosphoribosyl-1-pyrophosphate from the yeast has been devised. Important fluctuations of the P-Rib-P2 pool were measured in S. pombe at different stages of growth, the maximum taking place at the start of the exponential phase, whereas no variations in the specific activity of the P-Rib-P2 synthetase could be observed during the growth. The P-Rib-P2 intracellular content in the mutants devoid of purine phosphoribosyltransferases, namely pur 1, dap 1 and pur 1, dap 1, was increased up to 5-fold as compared to the wild-type strain. 4. The effect of intracellular concentrations of P-Rib-P2, a substrate for phosphoribosyltransferases, on the uptake rate of purines has been studied: addition of formycin to the growth medium lowered simultaneously the P-Rib-P2 intracellular content and the uptake of adenine and guanine. 5. Although our results demonstrate the activating effect of phosphoribosyltransferase activities on the uptake of adenine and guanine, they do not support the hypothesis of a 'group translocation' mechanism.