1. Plasma adenosine, inosine and hypoxanthine concentrations were assayed in seven control subjects, five myoadenylate deaminase deficient (MADD) patients and six McArdle patients before and after ischaemic forearm exercise.
2. The plasma adenosine increase was very low in all test groups and there were no significant differences.
3. The MADD patients showed a significantly lower increase of plasma inosine and hypoxanthine after exercise as compared with the controls.
4. In the McArdle patients the increase in plasma inosine and hypoxanthine after exercise did not differ significantly from the values measured in the controls.
5. The ischaemic exercise test with measurement of plasma inosine and hypoxanthine might be of diagnostic value in MADD, but not in McArdle's disease.