The Collaborative International Dictionary of English v.0.48:

Rhythmic \Rhyth"mic\ (-m[i^]k), Rhythmical \Rhyth"mic*al\
   (-m[i^]*kal), a. [Gr. ????: cf. L. rhythmicus, F.
   rhythmique.]
   Pertaining to, or of the nature of, rhythm
   [1913 Webster]

         Day and night
         I worked my rhythmic thought.            --Mrs.
                                                  Browning.
   [1913 Webster]

   Rhythmical accent. (Mus.) See Accent, n., 6
   (c) .
       [1913 Webster]

WordNet (r) 3.0 (2006):

rhythmic
    adj 1: recurring with measured regularity; "the rhythmic chiming
           of church bells"- John Galsworthy; "rhythmical prose"
           [syn: rhythmical, rhythmic] [ant: unrhythmic,
           unrhythmical]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "rhythmic":
   alternate, antispastic, beating, cadenced, cadent, circling,
   cyclic, dactylic, epochal, even, every other, iambic, in numbers,
   in rhythm, intermittent, isochronal, measured, metric, metronomic,
   oscillatory, palpitant, periodical, pitapat, prosodic, pulsatile,
   pulsating, pulsative, pulsatory, pulsing, pyrrhic, reciprocal,
   recurrent, recurring, regular, rhythmical, rotary, scanning,
   seasonal, serial, spondaic, staccato, steady, throbbing, trochaic,
   undulant, undulatory, wavelike, wheeling