The present invention relates to novel radiolabelled inhibitors of formula I for the Glycine 1 transporter (GlyT1), useful for the labelling and diagnostic imaging of the glycine 1 transporter functionality. wherein R1 is isopropoxy or 2,2,2-trifluoro-1-methyl-ethoxy and R2 is a radiolabelled group CH3, wherein the radionuclide is 3H or 11C. The radiolabelled compounds of formula I may be used as PET (Positron Emission Tomography) radiotracer for the labelling and diagnostic molecular imaging of the glycine 1 transporter functionality.
