CBF autoregulation during stepwise hypotensive hemorrhage in 12-month-old (n=9), 24-month-old (n=9), and 24-month-old melatonin-treated (0.39 mg kg−1 in drinking water, n=11) IcoWI rats (a) and in 13-month-old (n=8), 25-month-old (n=11), and 25-month-old melatonin-treated (0.44 mg kg−1 in drinking water, n=6) WAG/Rij rats (b). CBF values (percent baseline±s.e.m.) are grouped by mean arterial pressure ranges of 10 mmHg (20–29 to 120–129 mmHg) (Barry et al., 1982). *P≤0.05 vs baseline in the same group.