Intracytoplasmic sperm injection (ICSI) and embryo transfer (ET) in nonhuman primates, e.g. rhesus and cynomolgus monkeys, has been widely used in researches of reproductive and developmental biology, and the success rate has been improved significantly. However, unwanted multiple pregnancy occurs frequently during the ICSI-ET in monkeys, most of which leads to miscarriages. To improve the birth rate of pregnancies and to safeguard health of host and baby monkeys, multifetal pregnancy reduction (MPR) is necessary. In this study, a total of 10 monkeys with multiple pregnancies received MPR through transabdominal ultrasound-guided potassium chloride injection into beating hearts of selective fetuses. To assess MPR efficiency, 31 monkeys with normal singleton pregnancies and 25 monkeys with twin pregnancies without MPR were used as controls. The aim of the reduction is to keep only one fetus, no matter twin or triplet pregnancy originally. Our results show that six cases of MPR were successful and all of them retained single fetus. Moreover, about 1 month (30.2 +/- 1.2 days) of gestation is a better timing for MPR than later stage (50.7 +/- 1.9 days). We also found that the remaining fetuses developed normally with full-term gestation and normal birth weight. In conclusion, transabdominal ultrasound-guided potassium chloride injection is a safe and effective MPR method for monkeys with multiple pregnancies. Summary Sentence Transabdominal ultrasound-guided potassium chloride injection is a safe and effective multifetal pregnancy reduction method for monkeys with multiple pregnancies.