The efficiency in rats will differ from that in mice, but in general nanoparticle reagents for siRNA in vivo delivery seem to work the best (see https://altogen.com/product/nanoparticle-in-vivo-transfection-reagent/). There are various protocols depending on the target tissue you are trying to silence a gene in, but broadly speaking there has been plenty of research published on in vivo siRNA delivery, which should help you find the optimal protocol for your research needs.
The efficiency in rats will differ from that in mice, but in general nanoparticle reagents for siRNA in vivo delivery seem to work the best (see https://altogen.com/product/nanoparticle-in-vivo-transfection-reagent/). There are various protocols depending on the target tissue you are trying to silence a gene in, but broadly speaking there has been plenty of research published on in vivo siRNA delivery, which should help you find the optimal protocol for your research needs.