Synthesis and resistivity measurements on URu 2 Si 2 -NpRu 2 Si 2 solid solutions are reported. U 1 - x Np x Ru 2 Si 2 systems, with x = 0.01, 0.1, 0.3, 0.5, 0.7 and 0.9, crystallize in the tetragonal ThCr 2 Si 2 -type structure. Resistivity measurements indicate a marked variation of the transport properties with regard to the Np substitution. For low Np dilution (≤ 0.1) considerable changes are observed from the x = 0 Cr-like anomaly and a Kondo-like minimum appears below 10 K. In the systems for 0.3 ≤ x ≤ 0.7, an increase of T N is clearly seen and the Cr-like anomaly destroyed in favor of a strong increase of the resistivity suggesting a gap opening. At higher Np-concentration the resistivity is very similar to the pure Np compounds but a large increase in the pure Np-compound 6 K anomaly is observed.