The Collaborative International Dictionary of English v.0.48:

Hearken \Heark"en\ (h[aum]rk"'n), v. i. [imp. & p. p.
   Hearkened (-'nd); p. pr. & vb. n. Hearkening.] [OE.
   hercnen, hercnien, AS. hercnian, heorcnian, fr. hi['e]ran,
   h[=y]ran, to hear; akin to OD. harcken, horcken, LG. harken,
   horken, G. horchen. See Hear, and cf. Hark.]
   1. To listen; to lend the ear; to attend to what is uttered;
      to give heed; to hear, in order to obey or comply.
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            The Furies hearken, and their snakes uncurl.
                                                  --Dryden.
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            Hearken, O Israel, unto the statutes and unto the
            judgments, which I teach you.         --Deut. iv. 1.
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   2. To inquire; to seek information. [Obs.] "Hearken after
      their offense." --Shak.

   Syn: To attend; listen; hear; heed. See Attend, v. i.
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The Collaborative International Dictionary of English v.0.48:

Hearken \Heark"en\, v. t.
   1. To hear by listening. [Archaic]
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            [She] hearkened now and then
            Some little whispering and soft groaning sound.
                                                  --Spenser.
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   2. To give heed to; to hear attentively. [Archaic]
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            The King of Naples . . . hearkens my brother's suit.
                                                  --Shak.
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   To hearken out, to search out. [Obs.]
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            If you find none, you must hearken out a vein and
            buy.                                  --B. Johnson.
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WordNet (r) 3.0 (2006):

hearken
    v 1: listen; used mostly in the imperative [syn: hark,
         harken, hearken]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "hearken":
   attend, attend to, auscultate, be all ears, bend an ear, bug,
   cock the ears, eavesdrop, examine by ear, give attention,
   give audience to, give ear, hark, hear, hear out, heed, intercept,
   lend an ear, listen, listen at, listen in, listen to, sit in on,
   tap, wiretap