[en] Measurements have been made of the pressure of the bcc phase of solid ³He at molar volumes from 24.2 to 23.2 cm³ and at temperatures from 25 to 0.5 mK. The solid undergoes an unusual antiferromagnetic ordering at a temperature that depends strongly on the sample density. This ordering is due to the unusually large exchange interaction in solid ³He, a factor of 10⁴ stronger than the nuclear dipole-dipole interaction. The large latent heat associated with the first-order transition has constrained the size of the sample. The pressure of 0.005 cm³ of solid ³He has been measured while the volume of the experimental cell was regulated to within 15 parts in 10⁶. The results provide information on the magnitudes and molar volume dependencies of the exchange rates, and suggest that a single physical process governs all aspects of the ordering transition