To determine the crystal fields of hexagonal RRh3 (R=Pr, Nd) compounds showing ferromagnetic transitions at TC=3 and 6 K, respectively, we plan to perform inelastic neutron scattering experiments with powdered samples. RRh3 crystallizes in the hexagonal CeNi3-type structure, where the R ions sit in two sites of 2d with D_3h and 4f with C_3v. In NdRh3, the magnetic entropy S_m reaches Rln2 at around TC, indicating the two-fold degeneracy of the Kramers doublet is fully released. On the other hand, S_m of PrRh3 at around TC is only 2/3 of Rln2, suggesting that magnetic order occurs only at the 4f site. Because the Pr ion is a non-Kramers ion with J=4, the ground state could be a singlet or a doublet. Moreover, even in the doublet case, only z-component of the magnetic moment is active. They possibly lead to a peculiar magnetic structure different from that of NdRh3.