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.
      [1913 Webster]

            The Furies hearken, and their snakes uncurl.
                                                  --Dryden.
      [1913 Webster]

            Hearken, O Israel, unto the statutes and unto the
            judgments, which I teach you.         --Deut. iv. 1.
      [1913 Webster]

   2. To inquire; to seek information. [Obs.] "Hearken after
      their offense." --Shak.

   Syn: To attend; listen; hear; heed. See Attend, v. i.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hearken \Heark"en\, v. t.
   1. To hear by listening. [Archaic]
      [1913 Webster]

            [She] hearkened now and then
            Some little whispering and soft groaning sound.
                                                  --Spenser.
      [1913 Webster]

   2. To give heed to; to hear attentively. [Archaic]
      [1913 Webster]

            The King of Naples . . . hearkens my brother's suit.
                                                  --Shak.
      [1913 Webster]

   To hearken out, to search out. [Obs.]
      [1913 Webster]

            If you find none, you must hearken out a vein and
            buy.                                  --B. Johnson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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