Despite their continuous turnover, sheep antral follicles are always regularly innervated. Thelocal production of neurotrophins is probably involved in the control of ovarian innervation. In thiscontext the present investigation was designed to evaluate the ability of sheep antral follicles toproduce neurotrophic factors. In the first part of the paper neurotrophic activity was measured infollicular fluid of sheep antral follicles of different size. Using an in vitro model the effect ofgonadotrophins on neurotrophin production was then evaluated. The levels of neurotrophic activityin conditioned medium or follicular fluid and the kind of neurotrophin produced were determinedby using the chicken embryo dorsal root ganglia test combined with an immunoneutralization step.Follicular fluid from medium-large follicles (.4 mm) contains high levels of NGF (240–250ng/ml), whereas the factor is nearly undetectable in small follicles (,3 mm) and in early atreticfollicles. Experiments in vitro based on the culture of follicle shells for 12 hr confirmed thatmedium-large follicles can produce NGF. The production is strictly dependent on gonadotrophinstimulation. When gonadotrophins were not added or were added separately, no detectable levelsof neurotrophic activity accumulated in medium. By contrast, in the presence of both LH and FSHthe production of NGF became apparent showing a clear dose-response behavior. In addition, thisproduction increased progressively with increasing follicle size from 4 to .5 mm up to values ofabout 60 ng/follicle, whereas follicles with a diameter of less than 3 mm were insensitive togonadotrophins stimulation and did not produce significant amount of NGF. The data presenteddemonstrate that sheep follicles produce relevant amounts of NGF as long as the correct hormonalmilieu is provided. Under these conditions the production of the NGF increases with increasingfollicle size. This may be responsible for the rapid innervation of the wall of growing folliclesand/or take part in other non-neural processes that are generally attributed to gonadotrophinstimulation. [...]