To facilitate nuclear reprogramming, somatic cells or somatic cell nuclear-transferred (SCNT) oocytes have been treated with the histone deacetylase inhibitor trichostatin A (TSA), or the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (5-aza-dC), to relax epigenetic marks of differentiated somatic cells. TSA-treated SCNT oocytes have increased developmental potential, but the optimal treatment period is unknown. Reduced methylation levels in somatic cells have no positive effect on SCNT oocytes, but the treatment of SCNT embryos with 5-aza-dC has not been investigated. We examined the effect of TSA treatment duration on the developmental potential of mouse SCNT oocytes and the effect of 5-aza-dC treatment on their in vitro and in vivo developmental potential. To determine the effects of TSA treatment duration, nuclear-transferred (NT) oocytes were cultured for 0 to 26 h with 100 nM TSA. SCNT oocytes treated with TSA for 8 to 12 h had the higher rate of development to blastocysts and full-term fetuses were obtained after treatment for 8 to 12 h. When oocytes were treated for 14 h and 26 h, blastocyst rates were significantly decreased and fetuses were not obtained. To examine the effect of 5-aza-dC, 2-cell stage SCNT embryos were cultured with 10 or 100 nM 5-aza-dC for 48 h to the morula stage and transferred. The potential of embryos treated with 5-aza-dC to develop into blastocysts was decreased and no fetuses were obtained after transfer. The findings demonstrated that long-term TSA treatment of SCNT mouse oocytes and treatment with 5-aza-dC inhibit the potential to develop into blastocysts and to fetuses after transfer.