The biodistribution of N-isopropyl-p-[123I]iodoamphetamine (I-123 IMP) in the Macaca fascicularis monkey was determined at 15 min and at 1, 4, 24, and 48 hr after intravenous injection. Brain uptake was 7.8% of the injected dose at 1 hr, with little change in concentration between 15 min and 1 hr, falling thereafter. Eye uptake reached a maximum of 0.23% of injected dose at 24 hr, with activity primarily in the pigmented layers. The human absorbed radiation dose was calculated on the basis of biodistribution data. The critical organ is the eye (0.407 rad/mCi of I-123 IMP). The eye dose increased to 1.11 rad/mCi with 4% contamination from I-124 IMP and to 0.535 rad/mCi with 0.4% contamination from I-125 IMP. The absorbed dose to the liver was 0.127 rad/mCi for pure I-123 IMP and the thyroid dose was 0.120 rad/mCi, both increasing with either I-124 or I-125 contamination. While delayed eye uptake has not yet been reported in the human, care should be exercised in limiting the amount of contaminating I-124 or I-125 to the lowest practical level.