We have measured the pressure dependence of theFe57nuclear magnetic resonance frequency&ngr;in ferromagneticiron from1–10 000 kg/cm2at −77°C, 0°C, and 84.2°C. From these measurements the volumedependence of&ngr;has been obtained, enabling us to correct measurements of&ngr;(T)P = 1 atmfor the effects ofthermal expansion. By making a similar correction for the effect of thermal expansion on the temperaturedependence of the saturation magnetization&sgr;, we have found that, at constant volume, the hyperfinecoupling constantAin the relation&ngr; = A&sgr;is an explicit function of the temperature. Thus, measurementsof the temperature dependence of&ngr;, even when corrected to constant volume, do not give accurately thetemperature dependence of&sgr;. The results of a theory based on Stoner's collective electron model for thedelectrons are presented to account for the temperature dependence ofA.