This study investigated the parthenogenetic activation efficiency of oocytes from expanded (Ex) and compact (Cp) type of cumulus oocyte complexes (COCs) in the breeding and non-breeding seasons of horses (Equus caballus). The somatic cell nuclear transfer (SCNT) using fibroblasts from foals and adult horses was also studied. The results of parthenogenetic activation test showed that the proportions of Ex type oocytes developed to 2-cell, 4-cell and morula stages were 52.8% (19/36), 38.9% (14/36) and 5.6% (2/36) respectively, while those of Cp type oocytes were 47.9% (23/48), 33.3% (16/48) and 6.2% (3/48) respectively in the breeding season. The proportions of Ex type oocytes developed to 2-cell and 4-cell embryos were 37.2% (16/43) and 16.3% (7/43) respectively, while those of Cp type oocytes were 35.1% (27/77) and 11.7% (9/77) respectively, but morula stage embryos were not obtained in non-breeding season (Fig.4; Table 1). There was no significant difference in the rate of embryonic development between Ex and Cp (P > 0.05) groups in the same season, but there was significant difference between Ex and Cp groups in breeding and non-breeding seasons (P < 0.05). The percentages of foal fibroblast-derived SCNT embryos developed to 2-cell, 4-8 cell and morula stages were 41.5% (22/53), 33.9% (18/53) and 15.1% (8/53) respectively, while the percentages of adult horse fibroblast-derived SCNT embryos developed to 2-cell and 4-8-cell stages were 38.9% (7/18) and 22.2% (4/18) respectively, but no morula stage embryos were obtained (Fig.5; Table 2). In summary, season and COCs type affect the efficiency of parthenogenetic activation of horse oocyte, and different sources of nuclear donor cells affect the success rate of cloned embryo development.