A series of deformation tests was performed on samples from natural xenoliths and dunites with well-defined textures and microstructures. The rock samples were deformed perpendicular to the foliation, 45¿ to the foliation, and parallel to the foliation at temperatures from 1000 ¿C to 1200 ¿C, at a confining pressure of 300 MPa, and a constant strain rate of 10-5/s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material. The experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain. In all samples, both brittle and plastic mechanisms were operative. We observed nonlocalized-dilatant and localized-dilatant deformation features. Nonlocalized and localized plastic deformation created subgrain boundaries, deformation lamellae, and an increase in dislocation density in the olivine crystals. Heterogeneous dislocation activities occurred on the <100> (0kl) system at 1000 ¿C, and on the <100> (010) and <101> (010) systems at 1200 ¿C. Strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems. The postdeformation fabric intensity was lower in specimens with strong initial petrofabrics. Interactions between dislocations of different type led to high work hardening rates during high-temperature deformation, favoring intragranular microcracking. Fractures opened parallel to the <100> screw dislocations in the (010) plane, and in the {hk0} planes oblique to the (100) plane. The strength of the deformed samples was temperature and orientation dependent. Rocks deformed perpendicular to the foliation are strongest, and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45¿ to the foliation. Based on the laboratory results, we find important implications for the rheology of the lithosphere: (1) Peridotites develop semibrittle rheology even at high temperature, and show characteristic microstructures and fabric changes. (2) The orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure, temperature, and strain rate are fixed. (3) Interaction of slip systems produces strong work hardening and intragranular microcracking. ¿ 1998 American Geophysical Union