Abstract. For developmental competence of porcine embryos in vitro, it is important to improve the culture environment. The present study was performed to evaluate four different culture systems for in vitro matured porcine oocytes following intracytoplasmic sperm injection (ICSI); drop, well and two sizes of the well of the well (WOW) systems (500 and 1,000 µm in diameter). The cleavage rate on Day 2 and the mean cell number in blastocysts on Day 6 were not significantly different among the four treatments. However, the 1,000 µm WOW (24.6%) resulted in a significantly higher (P<0.05) blastocyst rate than those in the other culture systems (12.9, 14.8, and 7.1% for drop, well, and 500 µm WOW, respectively). The present study indicates that the microenvironment created by the 1,000 µm diameter WOW improves blastocyst production of in vitro matured porcine oocytes after ICSI, and that the effectiveness of the WOW system is dependent on the size (diameter) of the WOW. Key words: In vitro culture, Microenvironment, Porcine embryos, WOW (J. Reprod. Dev. 51: [533][534][535][536][537] 2005) n vitro production (IVP) for porcine oocytes has advanced remarkably during the past decades. T h e i n v i t r o c u l t u r e ( I V C ) s y s t e m f o r preimplantation embryos is an important step for IVP. Recently, it has been noted that the in vitro production techniques for pigs can be applied to o t h e r r e p r o d u c t i v e t e c h n o l o g i e s , s u c h a s production of alternative organs and specific proteins for humans through the modification of genes [1]. However, the proportions of blastocyst development and pregnancy rates of in vitroproduced embryos after transfer are usually lower than that of in vivo produced counterparts [1].In general, embryonic development in vitro is affected by a number of factors, such as co-culture w i t h s o m a t i c c e l l s [ 2 ] , s u p p l e m e n t w i t h antioxidants [3] and growth factors [4], and oxygen tension [5]. Moreover, another important factor is embryo density during culture [6,7], which influences the interaction of embryonic factors in the microenvironment. In vivo, embryonic factors are secreted by the embryo itself and appropriate c o n c e n t r a t i o n s a r e m a i n t a i n e d i n t h e microenvironment surrounding the embryo. The embryonic factors enhance the development of embryos through the autocrine-paracrine action. For improving the developmental competence of embryos in vitro, it is important to provide a culture environment similar to the in vivo conditions. Vajta et al. [8] developed the well of the well (WOW) system as a new culture system. The WOW created at the bottom of a four-well dish p r o v i d e s a m o r e c o n s t a n t a n d s u i t a b l e microenvironment for embryos. The open condition of the WOW may also provide an appropriate method of nutrition and dilution of