The invention relates to substituted piperidine, piperazine, morpholine and thiomorpholine compounds useful in the treatment of Alzheimer's disease and more specifically to compounds that are capable of inhibiting beta-secretase, an enzyme that cleaves amyloid precursor protein to produce amyloid beta peptide (A-beta), a major component of the amyloid plaques found in the brains of Alzheimer's sufferers. One embodiment of the invention relates to compounds having the structure: ##STR00001## where Z is CR or N, and R.sub.1, R.sub.2 and R.sub.3 are as described in the specification.