Short-term administration of IL-7 similarly affects cell numbers, cell turnover, and TREC content in thymus-intact and thymectomized mice. Mice were given rmIL-7 by continuous infusion at a dose of 5 μg per day, or diluent only, for 14 days. Total combined spleen and lymph node cell number (A) and TREC number (B), naive CD4⁺ cell number, defined by CD44^lo and CD45RB^hi coexpression (C), and CD8⁺ cells, defined by CD44^lo and CD103^hi coexpression (G), were determined. TREC frequency (number of TRECs/50 cells) was measured by real-time quantitative PCR (E,I), and the total TREC number in sorted naive CD4⁺ (F) and naive CD8⁺ (J) cells was calculated. To assess changes in cell turnover as a result of rmIL-7 administration, rmIL-7- or diluent-treated mice were given BrdU in the drinking water and the percentages of BrdU⁺ naive (CD44^lo) CD4⁺ (D) and CD8⁺ (H) cells were measured at the indicated time points. Data in panels A and B are data from a single experiment, representative of 2 experiments carried out, showing the mean plus or minus the standard deviation (SD) of 6 mice per group. Data in panels C through J are representative data from single experiments, representative of 2 experiments carried out, showing the mean plus or minus SD of 3 mice per group. ^*P < .05 when comparing rmIL-7 treatment to diluent control by the Mann-Whitney U test. ^**P = .09.