To determine the best developmental stage of donor embryos for yielding the highest number of clones per embryo, we compared the efficiencies of nuclear transfer when using blastomeres from morulae or morulae at cavitation, or when using inner-cell-mass cells of blastocysts as nuclear donors. This comparison was done both on in vivo-derived and in vitro-produced donor embryos, In experiment 1, with in vivo-derived donor embryos, nuclei from morulae at cavitation supported the development of nuclear transfer embryos to the blastocyst stage (36%) at a rate similar to that of nuclei from morulae (27%), blastomeres from morulae at cavitation being superior (P < 0.05) to inner-cell-mass cells from blastocysts (21%). The number of blastocysts per donor embryo was significantly (P < 0.05) higher when using nuclei from morulae at cavitation (15.7 +/- 4.1) rather than nuclei from morulae (9.8 +/- 5.5) or blastocysts (6.3 +/- 3.3). With in vitro-produced donor embryos (experiment 2), nuclei from morulae yielded slightly more blastocysts (32%) than nuclei from morulae at cavitation (29%), both stages being superior to nuclei from blastocysts (15% development to the blastocyst stage). Morulae at cavitation yielded a higher number of cloned blastocysts per donor embryo (11.5 +/- 5.9) than did morulae (9.3 +/- 3.2) and blastocysts (3.3 +/- 1.4). Transfer of cloned embryos originating from in vivo-derived morulae, morulae at cavitation, and blastocysts resulted in four pregnancies (10%), three pregnancies (7%), and one (17%) pregnancy on day 45. The corresponding numbers of carves born were 3 (4%), 3 (7%), and 0, respectively. After transfer of blastocysts derived from in vitro nuclear donor morulae (n = 16) and morulae at cavitation (n = 7), two (20%) and two (50%) recipients, respectively, were pregnant on day 45. However, transfer of seven cloned embryos from in vitro donor blastocysts to three recipients did not result in a pregnancy. Using in vitro-produced donor embryos, calves were only obtained from morula-stage donors (13%). Our results indicate that the developmental stage of donor embryos affects the efficiency of nuclear transfer, with morulae at cavitation yielding a high number of cloned blastocysts. (C) 1996 Wiley-Liss, Inc.