A model system of hamster oocyte and human sperm interaction was used to assess sperm entry into the perivitelline space after partial zona dissection. The procedure of zona dissection was standardized by creating slits which included one fourth, one eighth, one eighth X 2, or one sixteenth of the zonal circumference. Manipulated eggs were allowed to interact with 1 X 10(6) sperm/mL for 3 hours. A single large or medium slit was equally effective in permitting sperm entry into 46% and 47% of the manipulated eggs, respectively. However, the longer slit doubled the average number of sperm detected in the perivitelline space. A second medium-sized slit increased the rate of sperm entry into the perivitelline space to 76%, but the incidence of damaged oocytes also increased. A small slit did not permit sperm entry into any of the manipulated oocytes. This heterologous system of gamete interaction provides a model to evaluate requirements for successful partial zona dissection or other related procedures for assisted fertilization in the human.