The Similkameen batholith is an eccentrically zoned, Jurassic intrusive mass consisting marginally of older dioritic rocks and centrally of relatively younger granodioritic to granitic rocks. Seven plutonic rock units have been recognized and mapped. Petrochemical variations within the intrusive succession indicate that the rocks are of probable cogenetic origin. Petrochemical trends may be explained in terms of magmatic differentiation involving fractionation of hornblende and intermediate plagioclase. Rb, Sr, Rb/Sr, Ca/Sr, K/Rb, and Sr87/Sr86 values indicate that the intrusive succession has undergone a considerable degree of chemical evolution.