The proton spin–lattice relaxation rate has been investigated as a function of pressure (p≤250 MPa), temperature (180 K≤T≤300 K), and oxygen‐17 enrichment (25.7 atom‐%, 50.7 atom‐%) in metastable water. Intramolecular proton–oxygen 17 dipolar interactions in light water in relation to deuterium electric quadrupole interactions in heavy water carry similar dynamic information and allow the complicated dynamic isotope effect in liquid water to be investigated in the whole metastable range. Under hydrostatic pressure the proton relaxation rate curve R1 (T, p, ωH) has been investigated at five different Larmor frequencies (100 MHz≤ω≤500 MHz). These experiments unequivocally prove the isotropy of reorientational motions of water molecules on a nanosecond time scale and demonstrate the necessity to include fast quasilattice vibrations into a consistent interpretation of the relaxation rates in the dispersion regime.