Twenty-six male volunteers received a single oral dose of testosterone as free crystals or as the undecanoate ester. The latter was given either in crystalline form or in arachis oil. All preparations were tested three times in the same individual, whilst fasting on 2 days and on one day together with a breakfast rich in fat. Serum testosterone concentration was measured at intervals for up to 6--24 h after the dose. A significant and reproducible rise in serum testosterone level was found after ingestion of free testosterone. Testosterone esterified with undecylenic acid was only effective when administered in arachis oil. The meal increased the bioavailability of the ester, but had hardly any effect on that of the free hormone. It is concluded that bioavailability of oral testosterone can be improved by pharmaceutical means to an extent sufficient to produce adequate blood levels in substitution therapy.