Thymosin beta 4 (T beta 4) is a peptide of 43 amino acids that was first isolated from the thymus gland and subsequently found to be ubiquitous in nature. T beta 4 functions mainly as an actin-sequestering molecule in nonmuscle cells, where its primary role is to maintain the large pool of unpolymerized G-actin in the cell. Studies on the pharmacokinetics of T beta 4 in human and other mammals have not been reported so far. In the present study, we have measured T beta 4 concentrations in serum, urine, and 10 major organs of female Swiss-Webster mice following intraperitoneal administration of 400 micrograms synthetic T beta 4. Using a modified enzymatic immunoassay, our data show a significant increase of T beta 4 in serum starting 2 min after injection and lasting for 40 min (average: 2.34 +/- 0.54 micrograms/ml). High concentrations were found in urine (59.3 +/- 7.54 micrograms/ml) at three different points after injection (20 min, 40 min, and 2 h). Of the 400 micrograms T beta 4 administered to mice 83% was recovered at the end of the study, 44.6% of which corresponded to urine, 1.4% to serum, and 37.5% to the organs. In 50% of the tested organs, the wet weight concentrations of T beta 4 increased significantly from the first 40 min to 2 h after injection in comparison to their baseline wet weight concentrations. These organs were: the brain (72 micrograms/g), heart (80 micrograms/g), liver (15 micrograms/g vs 9 micrograms/g), kidneys (65 micrograms/g vs 28 micrograms/g), and peritoneal fat (47 micrograms/g vs 13 micrograms/g). Wet weight concentrations increased in the thymus (196 micrograms/g vs 147 micrograms/g) and muscle (45 micrograms/g vs 0 micrograms/g) after 6 h of injection. The spleen showed an increase in wet weight concentrations at the 2 min timepoint (267 micrograms/g vs 161 micrograms/g). Ovaries had a biphasic increase at 40 min (72 micrograms/g vs 62 micrograms/g) and 24 h (92 micrograms/g vs 62 micrograms/g) after T beta 4 administration. In lungs, the highest wet weight increase after injection (149 micrograms/g at timepoint 6 h) was not higher than its basal wet weight concentration (153 micrograms/g). These pharmacokinetic studies of T beta 4 in mice have established that high levels of T beta 4 are found in blood following I.P. administration and the kidney rapidly removes the peptide from the circulation. The kinetics of this response should help define the proper scheduling of administration of T beta 4 during clinical trials in disorders, such as the acute respiratory distress syndrome (ARDS), associated with actin toxicity.