One of the enantiomers of 5,5-dimethylmorpholinyl-2-acetic acid is observed to be a GABA_B receptor antagonist. The absolute configuration of the inactive enantiomer is found to be the R configuration. The morpholine ring adopts a chair conformation with the acetic acid moiety in an equatorial position. In the crystal packing, hydrogen bonds are observed between the ammonium group and the chloride ions.