We investigated the radiolabeling efficiency, in vitro stability, and biodistribution of radioactive iodine (I-131)-labeled 4-benzoyl-1-(4-carboxyphenyl)-5-phenyl-1H-pyrazole-3-carboxylic acid (P3CA). A quality-control study of the labeled substance (I-131-P3CA) was conducted using electrophoresis and radio thin-layer chromatography (RTLC). Biodistribution studies were undertaken in 9 female albino Wistar rats. Rats were killed at various times (15, 60 and 180 min), their organs removed, and percentage of injected dose per gram (ID%/g) values calculated. The labeling yield of P3CA was 97.08%+/- 0.80%. The maximum uptake of I-131-P3CA was seen in the lungs, stomach and spleen at 15 min. The uptake of labeled compound decreased over time in the lungs, whereas that in the stomach decreased. These data suggest that I-131-P3CA had high binding efficiency, high uptake in the lung, and sufficient stability to be used in diagnostic studies.