The present experiment aimed to determine the effects of supplements of folic acid (FA) alone or in combination with vitamin B12 on folate and homocysteine metabolism in gestating nulliparous Yorkshire–Landrace (YL) and multiparous Landrace (LD) occidental sows and multiparous Chinese Meishan–Landrace (ML) sows. LD sows were randomly assigned to two treatments: 0 or 15 mg FA/kg diet while YL and ML sows were assigned to three treatments: 0 mg FA/kg diet, 15 mg FA/kg or 15 mg FA+160 μg vitamin B12/kg diet. Supplements were given from the oestrus preceding insemination up to slaughter on day 15 of gestation. At slaughter, a uterine flush was collected to determine uterine contents of homocysteine, methionine, tetrahydrofolate (THF), 5-methyl-THF, pyridoxal 5-phosphate (P5P) and vitamin B12. Blood samples were taken at first oestrus, at insemination and on days 5, 10 and 15 of gestation to determine plasma concentrations of homocysteine, methionine, THF, 5-methyl-THF, P5P, vitamin B12 and relative total folate-binding capacity. In occidental sows (YL and LD), the FA supplement tended to decrease uterine flush content of homocysteine (P=0·06) and concentrations of plasma homocysteine (P=0·09). Nulliparous YL sows had lower concentrations of plasma homocysteine, methionine, THF and 5-methyl-THF (P<0·05) than multiparous LD sows. Multiparous ML and LD sows had similar concentrations of plasma THF, 5-methyl-THF, methionine and vitamin B12, but ML sows had lower concentrations of plasma homocysteine (P<0·05). The vitamin B12 supplement increased concentrations of plasma vitamin B12 (P<0·05) both in multiparous ML and nulliparous YL sows, but had no effect on the composition of either uterine flush or plasma. The present results showed also that sows had a low vitamin B12 status (<200 pg/ml) and high circulating homocysteine levels (>15 μM) during the first 15 d of gestation. Furthermore, the vitamin B12 content in uterine secretions represented between 180 and 300 % of the total content in plasma. The low plasma concentrations of homocysteine in multiparous ML sows suggest a more efficient remethylation pathway which may not be dependent upon dietary supply of FA or vitamin B12. In nulliparous YL sows, low concentrations of both homocysteine and methionine suggest that the methionine requirement for protein deposition might have reduced the amount of methionine available for the methylation pathway. The results of the present experiment suggest that the reduction of uterine homocysteine may be an important aspect of the role of FA supplement on the uterine environment in occidental sows. The presence of high levels of vitamin B12 in uterine secretions merits further investigation in relation to embryonic development.