Kidney silver concentrations ( n g/g tissue wet weight) before and after cessation of silver nanoparticle administration. The rats were allowed to recover for 1, 2, and 4 months following 28 days of oral silver nanoparticle exposure. **, P < 0.01 28-day exposure vs. recovery; *, P < 0.05 28-day exposure vs. recovery; a, P < 0.05 1-month recovery vs. 2-month recovery; b, P < 0.05 1-month recovery vs. 4-month recovery. A. Male (10 nm). B. Female (10 nm). C. Male (25 nm). D. Female (25 nm).