The aim of this study was to determine the effects of injecting 5 mg of PGF2alpha into the vulvar mucosa on the reproductive performance of sows maintained under field conditions. At an intensively managed piggery in northwest Spain, two experimental groups were formed randomly and observed throughout the year. The first group comprised sows receiving simultaneously, with every insemination, 5 mg of PGF2alpha injected into the vulvar lips. Group 2 sows received 1 ml of saline solution injected into the vulvar lips at insemination and served as the controls. The farrowing rates for each group were 78.46 and 54.39, while the litter sizes were 10.72 +/- 0.27 and 9.14 +/- 0.47 during the low fertility season (July-September). During the rest of the year (October-June), the farrowing rates were 83.91 and 80.93, while the litter sizes were 11.16 +/- 0.15 and 9.99 +/- 0.15. We conclude that injection of 5 mg of PGF2alpha into the vulvar lips at insemination is an effective method of compensating for the low fertility together with the decreased litter size of the summer months.