After oral administration of 4′‐chloro‐2‐ethylaminopropiophenone to man, unchanged drug and the de‐ethylated, reduced and reduced deethylated compounds were found in urine. Comparative urinary excretion studies after the oral administration of 4′‐chloro‐2‐ethyl‐aminopropiophenone in a sustained release form and in single or divided doses indicates that, when kidney tubular reabsorption is minimized, the biological availability of a drug can be followed by examining the excretion of either unchanged drug or metabolites formed quickly and directly from the administered drug. 1969 Royal Pharmaceutical Society of Great Britain