(h, i) Free thiol group (RSH) concentration in serum (h) and intracellular spermidine concentration in hepatocytes (i) of ageing mice with (open bars) or without (closed bar) supplementation of drinking water with spermidine (0.3 and 3 mM) for 200 days. Data represent mean ± s.e.m. (n = 3; *P 0.05 and **P 0.01). 