The longitudinal proton relaxation times T1 of the water protons have been determined at 100.1 MHz in the temperature range from +20 to −87 °C and up to pressures of 2500 bar. At temperatures below +10 °C, the T1 isotherms exhibit a maximum at pressures between 1.5 and 2 kbar. While at +10 and 0 °C, T1 rises by ∼10% from its atmospheric pressure value, this maximum becomes much more pronounced between −20 and −45 °C. In this region application of pressure increases T1 by ∼100%. The isobars at 2 kbar and above run through a minimum at −76 °C, indicating that at this temperature ωτϑ?1 and that the proton relaxation rate cannot be described by the extreme narrowing condition below ∼−40 °C. The experimental T1 data and the τϑ values derived at 2 kbar could be treated by a sum of two exponentials. While the smaller activation energies derived from this fit of 3.44±0.17 kcal mole−1 is independent of pressure, the higher activation energy decreases from 13±0.65 kcal mole−1 at atmospheric pressure to 9.3±0.5 kcal mole−1 around 1 kbar and then remains independent of pressure to 2.5 kbar. The data are qualitatively discussed in terms of a random hydrogen‐bond network.