Nymphs of Ornithodoros moubata were fed tritiated ecdysteroid. These ingested hormones are conjugated to fatty acyl esters that accumulate in the midgut (Connat et al. 1988). A few months later, the same ticks which had molted, were fed on physiological medium without radiolabel. At the issue of the blood meal, the nutritive medium contained an amount of radiolabel corresponding to 0.5% of the total labelling in the animal before the blood meal; this corresponded to 1.3% of the midgut content. These results demonstrate that in addition to transmission of parasites by saliva and coxal fluid (Burgdorfer 1951), transmission through regurgitation of the blood content in the gut could occur. An equivalent quantity of radiolabel was also emitted in the feces during and after the meal, but no conclusion about parasite transmission can be drawn from these "metabolic" results.