We applied high-frequency electron paramagnetic resonance to trinuclear 4f-3d heterometallic complexes, [{Ln(hfac)3}2{Ni(dpk)2(py)2}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter J(Ln-Ni) as well as nickel(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as J(Gd-Ni)/kB = +0.301(4) K, J(Tb-Ni)/kB = +0.216(12) K, and J(Ho-Ni)/kB = +0.110(3) K (defined as -J(Ln-Ni)∑J(Ln)(z)S(Ni)).