The gastrointestinal absorption of 244Pu(IV) has been measured in three male adult volunteers. The plutonium was in citrate solution and was taken with food. The work was carried out in two stages. The first stage measured urinary plutonium excretion up to 8 or 9 d after the oral intake. The second stage commenced about six months later with an intravenous injection of plutonium citrate and measurements of the urinary plutonium excretion. Results from the two routes of intake were used to calculate the fractional absorption (f1) of ingested plutonium. The f1 values were in the range (2-9) x 10-4. In theory it should be possible to measure the plutonium in the volunteers' urine throughout their lives. Measurements are continuing and the results show the excretion pattern up to nearly 2 y for one subject, and 6 months for the other two volunteers.