The QT interval and heart rate were measured from the electrocardiogram of 1608 Alderley Park beagles, and mean QT intervals for 28 values of heart rates between 84 and 192 beats per minute were calculated. These were analysed by linear regression and ranked by Pearson's correlation coefficient for each of eight equations, all of which have been proposed to describe the relationship between QT interval and heart rate. The equation QT=b(√RR), upon which the calculation of QTc using Bazett's formula is based, did not adequately describe the relationship. QT=b(3√RR) was a more adequate one-parameter equation. The firstranked equation was 1/QT=a+b(HR), but its use presents practical and interpretational difficulties; consequently, the second-ranked equation, QT=a+b(logeHR), is recommended for routine use.