Isolated atria from neonatal (0–5 day old) and adult (50 ± 5 day old) rats were perifused in oxygenated Kreb's Henseleit solution at 30 ± 0.5° C and exposed to four different concentrations of halothane, isoflurane, or enflurane while isometric contractile tension was recorded and compared with control atria. ED50 values (mM of anesthetic required to produce 50% reduction in contractile tension) of neonates for halothane (0.18 ± 0.01), isoflurane (0.41 ± 0.05), and enflurane (0.41 ± 0.04) were significantly lower than those of adults (0.35 ± 0.02, 0.80 ± 0.05, and 1.15 ± 0.05, respectively). Furthermore, neonatal ED50calculated as per cent of adult ED50was significantly less for enflurane (35%) than for halothane (54%) or isoflurane (51%).