MATERIALS AND METHODS Source of Drugs and Specimens. RAPA and “H-RAPA (174 mCi/mg) were gifts from Wyeth Ayerst Inc.(Princeton, NJ). RAPA standards were prepared in 50%(w/v) ethanol. The concentration of ethanol in all experiments described below was< 0.5%, which did not affect the distribution of the drug. Whole blood was collected from 10 normal subjects by venipuncture into tubes containing EDTA as anticoagulant. Specimens from a minimum of 4 individuals were pooled for each experiment. Plasma was obtained from the Red Cross. Temperature Equilibration Experiments. Whole blood was spiked with" H-RAPA (25,000 dpm/ml) and increasing amounts of RAPA to a final concentration of 5, 10, 25, 50, and 100 pg/L. The blood (initial temperature of either 4 “C or 22 C) was equilibrated for increasing lengths of time (0, 15, 30, 45, 60, 90, and 120 min) at 37 C before