We report the measurement of the C-13 NMR frequency in K3C60 under pre
ssure. Comparing pressure dependences of the hyperfine spin lattice re
laxation rate and of the resonance frequency provides the first determ
ination of the C-13 Knight shift ((61 +/- 4) p.p.m.) and of the chemic
al shift of the C-60(3-) molecule (125 p.m.m./TMS). NMR data under pre
ssure can be undestood with a weak-Coulomb-interaction model.