Elevated Mg2+ levels in the hemolymph ([Mg2+](HL)) of brachyuran crabs have recently been demonstrated to limit cold tolerance by reducing motor and circulatory activity. Therefore, the limiting function of elevated [Mg2+](HL) on circulatory performance and arterial hemolymph flow was investigated by the pulsed-Doppler technique in the spider crab Maja squinado during progressive cooling from 12°to 0°C. [Mg2+](HL) were reduced from control levels of 39.9 mmol L-1 to levels of 6.1 mmol L-1 by incubation in magnesium reduced seawater. At 12°C cardiac output was 13.9 ± 2.4 mL kg-1 min-1 and stroke volume 0.2 ± 0.04 mL kg-1 min-1 in control animals. In [Mg2+](HL)-reduced animals cardiac output increased to 43.6 ± 5.0 mL kg-1 min-1 and stroke volume rose to 0.6 ± 0.1 mL kg-1 min-1. Temperature reduction in control animals revealed a break point at 8°C linked to a major redirection of hemolymph flow from lateral to sternal and hepatic arteries. Cardiac output and heart rate dropped sharply during cooling until transiently constant values were reached. Further heart rate reduction occurred below 4.5°C. Such a plateau was not detected in [Mg2+](HL)-reduced animals where the break point decreased to 6°C, also indicated by a sharp drop in heart rate and cardiac output and the redirection of hemolymph flow. It is concluded that progressive cooling brings the animals from a temperature range of optimum cardiac performance into a deleterious range when aerobic scope for activity falls before critical temperatures are reached. Reduction of [Mg2+](HL) shifts this transition to lower temperatures. These findings support a limiting role for [Mg2+](HL) in thermal tolerance.