  Recent studies on the high-energy photons from gamma-ray bursts~(GRBs)
suggested a light speed variation $v(E)=c(1-E/E_{\mathrm{LV}})$ with
$E_\mathrm{LV}=3.6\times 10^{17}$ GeV. We check this speed variation from
previous observations on light curves of three active galactic nuclei (AGNs),
namely Markarian 421 (Mrk 421), Markarian 501 (Mrk 501) and PKS 2155-304. We
show that several phenomena related to the light curves of these AGNs can serve
as the supports for the light speed variation determined from GRBs.
