The in vitro culture of oviductal monolayers has proved to be an adequate model for studying sperm-somatic cell interactions and the secretory activity of the epithelia. However, the influence of oviductal fluid on oocytes has been much more difficult to mimic by using monolayers. However, oocyte preincubation in oviducts collected in the periovulatory period can overcome such limitations. The following study was intended (1) to assess the effect of the oviductal co-culture on the oocytes in relation to the sperm ability to fertilize them, and (2) to assess the effect of coculturing spermatozoa and oviductal cells from the isthmus and infundibulum on sperm fertilizing ability.Oviducts were collected about 48 h from the beginning of the estrus from regularly cycling goats slaughtered under general anaesthesia. Monolayers from isthmus and infundibulum were obtained by squeezing 4 cm of isthmus and 4 cm of infundibulum as described by Pollard et al (Biol. Reprod. 1991). The remnant of the oviduct was used for oocyte incubation experiments. The cells were washed and seeded in 4-well TC plates suspended in 199 supplemented with serum, insulin and Hepes. Later, they were incubated at 39°C under 5% CO 2 in air. Only confluent monolayers were used in the experiments.Cumulus-oocyte complexes were obtained from ovaries collected at a local abbatoir and matured in supplemented medium 199 at 39°C under 5% CO 2 in humidified air for 24 h. Later, denuded oocytes were suspended in Talp calcium free medium and either transferred to the oviducts ipsilateral to the ovulated ovaries or incubated in already prepared sperm suspensions. Fresh semen was collected by artificial vagina from Anglo Nubian bucks and capacitated by incubation and suspension in Talp plus heparin as described by Cox et al (Theriogenology, 1994). Oocytes were considered to be fertilized when pronuclei development and sperm tail could be observed.Experiment 1, studied the effect of oviductal co-culture with oocytes on fertilization rate. Oocytes were transferred to oviducts and then incubated for 30 min at 39°C under 5% CO 2 in air. After incubation, the oocytes were collected and transferred to 50 μl sperm suspensions. Experiment 2, studied the interactions between heparin, incubation and isthmus cells on the fertilizing ability of spermatozoa. Medium 199 was replaced by Talp either 4 or 24 h before the addition of 1.5 10 6 sperm/ml and heparin was added in the medium except for isthmus cells incubated for 24 h. Experiment 3, studied the effect of isthmus and infundibulum cells on the fertilization efficiency of spermatozoa (0.9 10 6 33 sperm/ml in 0.5 ml).The results showed that in Experiment 1, oviductal co-culture did not affect the efficiency of fertilization (incubated oocytes: 309434 (71.2%) vs non-incubated oocytes: 329406 (81.0%); P>0.05). In Experiment 2, incubating Talp medium in presence of heparin improved the effect of the isthmus cells on fertilization (no cells: 6875 (90.7%) a b ; isthmus-4h: 5872 80.6%) a ; isthmus-24h+heparin: 6771 (94.4%) b ; isthmus-24h: 1974 (25.7%) c ; a , b , c P<0.05). In Experiment 3, the co-incubation of spermatozoa with isthmus cells improved fertilization rates (no cells-24h: 313449 (69.7%) a ; isthmus-24h: 278348 (79.9%) b ; infundibulum-24h:273407 (67.0%) a ; a , b P<0.01).Collectively, these results suggest that the oviduct affects fertilization efficiency by improving the sperm performance but not the oocyte fertilizability (Grant Fondecyt 1940977).