There are a number of mammalian species with complicated or unknown mechanisms for sex determination. The literature is reviewed with special reference to the origin and evolution of the sex chromosome mechanisms in three species of monotremes and in the following species of rodents: the creeping vole (Microtus oregoni), the mole vole (Ellobius lutescens), the Amami spinous country rat (Tokudaia osimensis), the wood lemming (Myopus schisticolor), the varying lemming (Dicrostonyx torquatus), South American field mice (Akodon sp.), and the short-tailed bandicoot rat (Nesokia indica). Although this is a heterogeneous group with regard to their sex chromosomes, it is striking that among placental mammals only one order, Rodentia, and only two of its subfamilies, Microtinae and Murinae, are represented. Various reasons for this are discussed. Some of these species have proven to be excellent models for studies of fundamental processes involved in sex determination.