The effect of ethanol on nucleotide catabolism was studied in mouse purine nucleoside phosphorylase deficient (NSU-1) T lymphoblastoid cells. Cells were pre-incubated first with radioactive adenine to label intracellular ATP and then incubated with a range of ethanol concentration (0-5%). Changes in intracellular concentrations of nucleotides and amounts of nucleosides excreted into the medium were measured. It was found that ethanol induces ATP degradation which increased with increasing ethanol concentrations. However, no significant changes were found in ADP or AMP levels. Inosine was the major product of ATP catabolism excreted to the medium but small amounts of adenosine were also found. Inhibition of adenosine deaminase activity by deoxycoformycin increased only slightly the excretion of adenosine but had no effect on inosine production. This indicates that ethanol induced nucleotide catabolism proceeds mainly via AMP deaminase and IMP dephosphorylase reactions. Ethanol had no effect on the intracellular orthophosphate levels. On the other hand ethanol in low concentrations (below 1%) decreased adenine and uridine incorporation into nucleotides, and thymidine incorporation into DNA. The elevation of nucleotide catabolism together with the inhibition of nucleotide synthesis may significantly alter nucleotide availability for a number of cellular processes. It may therefore explain suppression of the blastogenic transformation and cause lymphopenia in acute alcoholism.