Abstract The heart-beat frequency of the American cockroach gives a linear plot against temperature over the range 12–40°C; below 12° the frequency deviates considerably from linearity. In intact cockroaches the heart continues beating well below the cold stupor temperature of the animal: to 1·8° in cold-adapted and to 4° in warm-adapted individuals. No differences were found between the sexes, and, except for the cold-inactivation temperatures, no differences between cold-adapted, room-adapted, and warm-adapted individuals. Also, no significant differences were encountered when the temperature was falling during the tests in contrast to when it was rising. Constraint or irritation of the specimen raises the values about 30 per cent. Semi-isolated heart preparations give the same linear relationship but are inactivated at 8–9°C. It is suggested that the automaticity resident in the heart is inactivated at 8–9° and supplanted at lower temperatures by stimulating impulses from the central nervous system. The same hypothesis could account for the deviation from linearity shown by intact cockroaches below 12°. Q 10 values rise gradually from 1·3 at the highest temperatures to 2·5 at the lowest temperatures. Correspondingly, the μ values of Arrhenius plots (calculated from tangents to the curve) change gradually from 7000 to 18,000. In the range of 20–25° the Q 10 is just under 2 and the μ value 12,000–13,000.