Tc-D,D- and Tc-L,L-HMPAO were synthesized. The stability of Tc-D,D- and Tc-L,L-HMPAO in vitro is similar to that of d,l-isomers by the spectrophotometric and three strips methods. Cerebral uptake (%D in brain) for the L,L-isomer is higher than the D,D- and d,1-isomer in rats. Delayed studies shows that TC-L,L-HMPAO reveals less washout and reflects a higher cerebral deposition properties than the D,D- and d,l-isomer. (C) 1999 Elsevier Science Ltd. All rights reserved.