The distribution of lysosomes during porcine oocyte maturation and fertilization in vitro was revealed by using acridine orange staining. The lysosomal distribution was classified into three types: generally even distribution throughout the ooplasm (Type I), less distribution in the peripheral ooplasm (Type II), and less distribution in the peripheral and inner ooplasm (Type III). All oocytes examined at 0 and 8 h after maturation culture showed the Type I lysosome distribution and 97% were at the germinal vesicle (GV) stage. When cultured for 22 h, the relative abundance of Type I oocytes decreased to 58%, while Type II and Type III oocytes appeared at relative abundances of 35 and 8%, respectively. After 32 and 44 h of culture, 35 and 80% of the oocytes, respectively, were Type III. When cultured with olomoucine, IBMX or dbcAMP for 22 h, 100, 79 and 94% of the oocytes, respectively, showed the Type I distribution of lysosomes, and their nuclei were almost all at the GV stage (100, 93 and 100%). The results of the present study suggest that there may be a close relationship between nuclear maturation and the distribution of lysosomes in the cytoplasm, and that the distribution of lysosomes may be one of the criteria of cytoplasmic maturation of porcine oocytes.