Magnetoresistance ͑MR͒ measurements on the antiferromagnetically ordered polycrystalline R 2 Ni 3 Si 5 ͑R ϭ Pr, Dy, Ho; T N ϭ 8.5 K, 9.5 K, 6.0 K, respectively͒ materials are reported here. Field dependence of MR, measured both below and above their respective ordering temperatures and up to a field of 45 kOe, is found to be anomalous. In all the three cases, in the magnetically ordered state, MR is not monotonic but exhibits a maximum and a minimum. A considerable contribution to MR, with nearly linear field dependence and persisting even above the magnetic ordering temperature, is observed, which is unusual. Temperature dependence of MR ͑in a constant high field͒ above the magnetic ordering temperature, measured up to 100 K, exhibits a pronounced hump. The behavior of magnetoresistance at low temperatures is interpreted in terms of metamagnetism along with a short range ferromagnetic interaction. ͓S0163-1829͑96͒05834-1͔The members of the R 2 Ni 3 Si 5 ͑R ϭ rare earths͒ series exhibit many unusual and interesting magnetic, electrical, and thermal properties. For example, in Ce 2 Ni 3 Si 5 and Eu 2 Ni 3 Si 5 , the rare earth ions are in valence fluctuating state, 1,2 Tb 2 Ni 3 Si 5 shows a double magnetic transition, 3 and magnetic ordering temperatures of many of the members of this series do not follow the well established deGennes scaling. [3][4][5] In order to understand the nature of magnetic interaction in these materials, we have carried out magnetoresistance ͑MR͒ measurements. Here we report our MR results on R 2 Ni 3 Si 5 ͑R ϭ Pr, Dy, Ho͒ which exhibit an anomalous temperature and field dependence.The synthesis and preliminary magnetic properties of R 2 Ni 3 Si 5 ͑R ϭ Pr, Dy, Ho͒ have been reported earlier. 3,5,6 These compounds crystallize in orthorhombic U 2 Co 3 Si 5 -type structure ͑space group Ibam͒. 6 The compounds Pr 2 Ni 3 Si 5 , Dy 2 Ni 3 Si 5 , and, Ho 2 Ni 3 Si 5 order antiferromagnetically with T N ϭ 8.5 K, 9.5 K and 6 K, respectively. High temperature susceptibility follows a Curie-Weiss ͑CW͒ behavior, with a magnetic moment close to the respective free trivalent R moment.Field dependence of MR, ⌬ / (0)"ϭ͓ (H)Ϫ (0)͔/ (0), where (H) is the resistivity measured at magnetic field H…, for Pr 2 Ni 3 Si 5 , Dy 2 Ni 3 Si 5 and Ho 2 Ni 3 Si 5 at different temperatures is shown in Figs. 1, 2, and 3, respectively. An expanded plot of magnetoresistance and magnetization at 6K for Ho 2 Ni 3 Si 5 at low field ͑0-5 kOe͒ is given in Fig. 4 for clarity. For the sake of comparison, MR of Y 2 Ni 3 Si 5 ͑where no magnetic effects associated with R ions are expected as Y is nonmagnetic͒ is also presented here ͑Fig. 5͒.The salient features of MR, common to all the three materials, Pr 2 Ni 3 Si 5 , Dy 2 Ni 3 Si 5 , and Ho 2 Ni 3 Si 5 , are described below taking Dy 2 Ni 3 Si 5 as a typical example ͑Fig. 2͒. ͑a͒ Below the magnetic ordering temperature ͑T N ϭ 9.5 K͒, MR first increases with increasing field, reaches a maximum (ϳ8%͒ at 12 kOe ͑at 4.4 K͒, then decreases and reaches a minimum around 35 kOe and then o...