Sea urchin (Lytechinus variegatus) eggs have been used to study the effects of fentanyl on in vitro fertilization and early development as a model of human in vitro fertilization. Fentanyl did not affect fertilization or subsequent cell division when present in concentrations calculated to approximate or exceed those to which human ora would be exposed during clinical anesthesia. Lytechinus eggs exposed to fentanyl, then washed in fentanyl-free sea water before fertilization were also not affected and divided normally. The results suggest potential clinical utility of fentanyl during the harvesting of human ova for subsequent in vitro fertilization.