High hydrostatic pressure (HHP) has been introduced into the field of embryology recently, with the possible mechanism that a sublethal HHP could induce the synthesis of molecular chaperons to protect the embryos from further stresses. Improved cryotolerance has been achieved successfully in HHP-treated mouse (Pribenszky 2005 Anim. Reprod. Sci. 87, 143–150) and bovine (Pribenszky 2005 Reprod. Domest. Anim. 40, 338) embryos, and the semen of bull (Pribenszky 2007 Reprod. Fertil. Dev. 19, 181–182) and boar (Pribenszky 2005 Reprod. Fertil. Dev. 18, 162–163). The objective of the present study was to apply this new technique to in vitro-matured (IVM) porcine oocytes and further investigate its effect in the procedure of handmade cloning (HMC). After 40 h IVM, cumulus–oocyte complexes (COCs) were loaded in 0.5-mL straws by a 2-mL syringe, with HEPES-buffered TCM199 as the loading medium. COCs were then treated with 20 MPa (200 times greater than atmospheric pressure) for 60 min by a pressurizing device (Cryo-Innovation Inc., Budapest, Hungary), with an interval of 120 min between HHP treatment and subsequent HMC. Two different cell lines (from Day 40 fetuses of Yucatan and Danish Landrace breeds (LW1-2)) were used as donor cells for nuclear transfer. A total of 592 reconstructed embryos were produced from both HHP-treated and control groups and were in vitro cultured for 6 days to evaluate the developmental competence through to blastocyst formation. The effect of donor cells on blastocyst development was also investigated. SPSS 11.0 program (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis; values with P < 0.05 were regarded as significant. Blastocyst rates of the different groups are shown in Table 1. Our results indicated that COCs treated with HHP had a much higher blastocyst rate than those untreated (P < 0.01) and this improvement was not affected by using different donor cells for nuclear transfer. In conclusion, the sublethal HHP treatment could improve the in vitro developmental competence of porcine IVM oocytes when they are used for HMC. Further in vivo experiments are required to investigate the long-term effect of HHP on embryo development.