Terminally radioiodinated long-chain fatty acids (FA) or phenyl-FA have shown promise for external myocardial imaging and detection of metabolic disorders with changes like those seen with radiocarbon-labelled natural FA. Yet, questions remain about the correlation of these changes to the biochemical forms of radioiodine within the cell. We have measured the distribution of label from 123I-heptadecanoic acid (HA), 123I-para-phenyl- and ortho-phenylpentadecanoic acid (pPPA and oPPA), 1-14C-stearic acid (SA) and 1-14C-palmitic acid (PA) into complex lipids (CL) (i.e. triacylglycerols (TG) and phospholipids (PL)), aqueous phase (AP) and solid residue (SR) of myocardium of normal (N) and streptozotocin-diabetic (D) rats at 1,2, 3, 5 and 10 min after i.v. injection. For the total heart (TH) all FA have similar initial peak heights in % dose (pPPA highest) at 1 to 2 min in N and D. Tracer kinetics for CL, TG and PL were similar for PA and pPPA and for SA and HA. For PA and pPPA 2/3 of CL tracer were in TG and 1/3 in PL, whereas it was 1/2 in each for SA and HA. In D rats turnover of CL was enhanced and tracer uptake into TG reduced for all FA. Tracer kinetics in SR and AP were similar, with highest values for HA; relationship between peak times for CL and AP indicates rapid formation of catabolite and its early cell exit. Unlike pPPA, oPPA was little converted to TG or PL and equally in N and D rats showed rapid decline.