Galectin-1 and galectin-3, beta-galactoside-binding lectins, are specifically expressed in the regressing corpus luteum (CL) of mice; however, their function remains unclear. In this study, we examined the effects of prolactin (PRL) and prostaglandin F-2 alpha (PGF(2 alpha)), two main regulatory molecules of mouse CL function, on galectin expression. In situ hybridization analysis clearly demonstrated an initial increase in galectin-1 in the newly formed CL (CLN) after postpartum ovulation 48 h after compulsory weaning. This was accompanied by a decline in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and LH receptor (LH-R) expression, suggesting a withdrawal of PRL stimulation. At 72 h after the weaning, the expression of both galectins in CLN was remarkably increased, being associated with an intense expression of progesterone degradation enzyme (20 alpha-HSD). Compulsory weaning did not significantly alter both galectin expression in the remaining CL of pregnancy (CLP), while PGF(2 alpha) strongly upregulated both galectin expression only in the remaining CLP, which lacked LH-R in postpartum mice. Administration of bromocriptine, an antagonist for PRL secretion, to nonpregnant cyclic mice induced an accumulation of galectin-1 - but not galectin-3 - in all CL of various generations, and additional PRL treatment reduced its accumulation, suggesting a direct suppressive effect of PRL on galectin-1 expression. Although the function and regulatory mechanism of galectin in the CL is not fully understood, PGF(2 alpha) is an excellent candidate that regulates galectin expression, but its effect may be abolished by LH-R-mediated signal. PRL withdrawal seems to be necessary for an initiation of luteolysis and the following PGF(2 alpha)-induced galectin expression. Reproduction (2012) 144 617-624