Electron transport and magnetic properties of several compositions of the La 1 - x Sr x - z Y z MnO 3 system have been investigated in order to explore the effect of yttrium substitution on the magnetoresistance and related properties of these manganates. Yttrium substitution lowers the T c and the insulator-metal transition temperature, while increasing the peak resistivity. A comparison of the properties of La 1 - x Sr x - z Y z MnO 3 with the corresponding La 1 - x Ca x - z Y z MnO 3 compositions shows that the observed properties can be related to the average size of the A-site cations.