The spin-averaged binding energy and the hyperfine mass splitting of heavy-light mesons are investigated within the constituent quark model as a function of the inverse heavy-quark mass. It is shown that the so-called heavy-quark kinetic energy, −λ12mQ, may differ remarkably from the non-relativistic expectation
2mQ, thanks to relativistic effects in the effective interquark potential for heavy-light mesons, which may yield substantial 1mQ corrections to the heavy-quark static limit. The determination of the difference of the hadronic parameter λ1 in the Bu(d) and Bc mesons can provide information about the strength of relativistic effects in the interquark interaction.