The present study examined in rats the concentration-time profiles of 2',3'-dideoxyinosine (ddI) in pharmacologically relevant tissues, including organs where drug effects are desired (brain, lymph nodes, spleen), organs with known drug toxicity (pancreas), and major eliminating organs (liver, kidney). ddI was analyzed by high performance liquid chromatography and radioimmunoassay. In the liver, pancreas, spleen, brain and lymph nodes, the highest concentrations were reached between 4 and 7 min after drug administration. The concentrations subsequently declined in parallel with those in plasma, indicating that plasma and these tissues were in rapid equilibrium. The concentrations in these tissues were less or equal to that of plasma. In the kidney, the maximal concentration occurred at a later time of 14 min, after which concentrations also declined in parallel to those in plasma. The kidney concentrations were about 10-fold greater than plasma concentrations. The ratios of tissue-to-plasma concentrations and of areas under the tissue and plasma concentration-time profiles showed a 230- to 300-fold range with the rank order of kidney >> liver approximately pancreas > lymph nodes > spleen >> brain, with respective values of 10.4, 1.09, 0.90, 0.75, 0.42, and 0.04. These data indicate no accumulation of ddI in brain, lymph nodes, spleen, pancreas and liver, and a significant accumulation in the kidney. The low tissue-to-plasma ratios in brain and spleen and a moderate ratio in lymph nodes indicate that further enhancement of the therapeutic effect of ddI requires improved drug delivery and entrapment in these tissues.