The invention provides a compound of formula (I) or a salt or solvate thereof: ##STR00001## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, n, Het and Ar are as defined in the specification, and uses of such compounds. The compounds inhibit GlyT1 transporters and are useful in the treatment of certain neurological and neuropsychiatric disorders, including schizophrenia.