  A test of lepton universality, performed by measuring the ratio of the
branching fractions of the $B^{0} \rightarrow K^{*0}\mu^{+}\mu^{-}$ and $B^{0}
\rightarrow K^{*0}e^{+}e^{-}$ decays, $R_{K^{*0}}$, is presented. The $K^{*0}$
meson is reconstructed in the final state $K^{+}\pi^{-}$, which is required to
have an invariant mass within 100$\mathrm{\,MeV}c^2$ of the known
$K^{*}(892)^{0}$ mass. The analysis is performed using proton-proton collision
data, corresponding to an integrated luminosity of about 3$\mathrm{\,fb}^{-1}$,
collected by the LHCb experiment at centre-of-mass energies of 7 and
8$\mathrm{\,TeV}$. The ratio is measured in two regions of the dilepton
invariant mass squared, $q^{2}$, to be \begin{eqnarray*} R_{K^{*0}} =
\begin{cases} 0.66~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm
0.03\mathrm{\,(syst)} & \textrm{for } 0.045 < q^{2} < 1.1~\mathrm{\,GeV^2}c^4
\, , \\ 0.69~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.05\mathrm{\,(syst)} &
\textrm{for } 1.1\phantom{00} < q^{2} < 6.0~\mathrm{\,GeV^2}c^4 \, .
\end{cases} \end{eqnarray*} The corresponding 95.4\% confidence level intervals
are $[0.52, 0.89]$ and $[0.53, 0.94]$. The results, which represent the most
precise measurements of $R_{K^{*0}}$ to date, are compatible with the Standard
Model expectations at the level of 2.1--2.3 and 2.4--2.5 standard deviations in
the two $q^{2}$ regions, respectively.
