  Rare $d$-electron derived heavy-fermion properties of the solid-solution
series LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ were studied for $1 \leq x \leq 4$ by
resistivity, susceptibility, specific-heat measurements, and magnetic-resonance
techniques. The pure ruthenate ($x = 4$) is a heavy-fermion metal characterized
by a resistivity proportional to $T^2$ at low temperatures $T$. The coherent
Kondo lattice formed by the localized Ru 4$d$ electrons is screened by the
conduction electrons leading to strongly enhanced effective electron masses. By
increasing titanium substitution the Kondo lattice becomes diluted resulting in
single-ion Kondo properties like in the paradigm $4f$-based heavy-fermion
compound Ce$_x$La$_{1-x}$Cu$_{2.05}$Si$_2$ [M. Ocko {\em et al.}, Phys. Rev. B
\textbf{64}, 195106 (2001)]. In LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ the
heavy-fermion behavior finally breaks down on crossing the metal-to-insulator
transition close to $x = 2$.
