In fully grown pig oocytes, meiotic maturation in vitro is slowed by inhibition of histone deacetylases by trichostatin A (TSA). In growing oocytes with partial meiotic competence, the culture with TSA has no significant effect on the meitic maturation. Growing oocytes treated with TSA mature mainly to the stage of metaphase I. The ratio of oocytes maturing to the stage of metaphase II is very limited. After the transient exposure to TSA, the maturation of growing oocytes with partial meiotic competence takes another course. When these oocytes are first cultured in TSA-free medium, then cultured for another 24 hours with 100 nM TSA and finally they are again cultured in TSA-free medium for 24 hours, the ratio of oocytes maturing to the stage of metaphase II is significantly increased and it reached 59%. When oocytes are cultured for the same time without transient exposure to TSA, only 19% matured to the stage of metaphase II.