The room‐temperature internal magnetic field Hi, electric quadrupole coupling ε, and isomer shift IS of divalent 57Fe in CoO have been measured as a function of pressure up to about 250 kbar. Radioactive samples were prepared by heating 57Co‐enriched Co3O4 in vacuum above 1000°C. The main features of the velocity spectra are qualitatively similar to those of the temperature study of this system as reported by Wertheim,1 except that the fraction of trivalent 57Fe observed is substantially reduced.2,3 As the pressure is raised the velocity spectrum unfolds into a magnetic hyperfine pattern in which Hi increases from zero to about 210 kOe at 250 kbar. This value is somewhat higher than the low‐temperature saturation value of approximately 180 kOe. The rate of increase of Hi with pressure, coupled with low‐temperature and x‐ray pressure‐volume measurements,4 shows that the Néel temperature TN (291°K at atmospheric pressure) increases with decreasing volume in a manner consistent with the results of Bloch et al.5 [d(lnTN)/d(lnV)≅ −3 at atmospheric pressure]. The velocity spectra also indicate small changes in both IS and ε with pressure, the former varying from −1.16 mm/sec at atmospheric pressure to −1.08 mm/sec at 250 kbar, relative to a stainless steel absorber. Both the change in IS and saturation value of Hi are consistent with an expansion of the 3d charge density and an accompanying increase in the 3s charge and spin density at the nucleus, because of less screening. The parameter, ε=¼e2qQ, in the antiferromagnetic phase is observed to approximately double from −0.03 mm/sec at 100 kbar to −0.06 mm/sec at 250 kbar. (These values are based on the unproven assumption that Hi is parallel to the axis of an axially symmetric electric field gradient tensor.)