The Collaborative International Dictionary of English v.0.48:

deteriorate \de*te"ri*o*rate\ (d[-e]*t[=e]"r[i^]*[-o]*r[=a]t),
   v. i.
   To grow worse; to be impaired in quality; to degenerate.
   [1913 Webster]

         Under such conditions, the mind rapidly deteriorates.
                                                  --Goldsmith.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Deteriorate \De*te"ri*o*rate\ (d[-e]*t[=e]"r[i^]*[-o]*r[=a]t),
   v. t. [imp. & p. p. Deteriorated
   (d[-e]*t[=e]"r[i^]*[-o]*r[=a]`t[e^]d); p. pr. & vb. n.
   Deteriorating (d[-e]*t[=e]"r[i^]*[-o]*r[=a]`t[i^]ng).] [L.
   deterioratus, p. p. of deteriorare to deteriorate, fr.
   deterior worse, prob. a comparative fr. de down, away.]
   To make worse; to make inferior in quality or value; to
   impair; as, to deteriorate the mind. --Whately.
   [1913 Webster]

         The art of war . . . was greatly deteriorated.
                                                  --Southey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deteriorate
    v 1: become worse or disintegrate; "His mind deteriorated"
    2: grow worse; "Her condition deteriorated"; "Conditions in the
       slums degenerated"; "The discussion devolved into a shouting
       match" [syn: devolve, deteriorate, drop, degenerate]
       [ant: convalesce, recover, recuperate]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "deteriorate":
   agent provocateur, aggravate, alter, ameliorate, amplify, annoy,
   augment, be changed, be converted into, be renewed, bottom out,
   break, build up, change, checker, chop, chop and change,
   come about, come around, come down, come round, crumble, damage,
   debilitate, decay, decline, decompose, deepen, degenerate, degrade,
   depreciate, descend, deviate, dilapidate, disimprove, disintegrate,
   diverge, diversify, embitter, endamage, enhance, enlarge, erode,
   exacerbate, exasperate, fade, fall back, flag, flop, get worse,
   go downhill, go to pot, grow worse, harm, haul around,
   have a comedown, heat up, heighten, hit rock bottom, hot up, hurt,
   impair, improve, increase, injure, intensify, irritate, jibe,
   languish, lessen, let down, magnify, make acute, make worse, mar,
   meliorate, mitigate, modulate, mutate, provoke, put back,
   reach the depths, regress, relapse, retrograde, retrogress, revive,
   rot, sharpen, shift, sicken, sink, slacken, slide, slip, slip back,
   sour, spoil, swerve, tack, take a turn, touch bottom, turn,
   turn aside, turn into, turn the corner, undergo a change,
   undermine, vary, veer, warp, weaken, worsen