The inhibitory effect of procaine hydrochloride and Gerovital-H3 on monoamine oxidase (MAO) activity in the rat brain was studied at six time points during the 24-hour cycle. It was found that the percent inhibition is time dependent. Both drugs showed maximum inhibition when the MAO activity was at the lowest values during the 24-hour period. In addition to time dependence, these drugs are also age dependent because the MAO activity varies with age.