The disappearance of aminopyrine from plasma was studied in man, comparing young and old, female and male subjects. It was shown that the rate of disappearance is higher in young than in old subjects, and that there is no difference between female and male subjects of the ‘old’ group. Within the same group, differences in plasma levels can be seen, depending on the pharmaceutical preparation used. In the case of one kind of aminopyrine tablet (aminopyrine B), the levels of aminopyrine in plasma did not decline during the entire experiment lasting of 9 h.