Prolactin (PRL) is recognized as a metabolic regulator during lactation, but little information exists on its actions in male adipose tissue. We examined whether PRL affects the expression of its receptors (PRLR), lipolysis, and adipokine secretion in male rats. Both long and short PRLR isoforms were induced 40-50-fold during differentiation of epididymal preadipocytes, with a 10-fold higher expression of the long isoform. PRL upregulated both isoforms before and after differentiation. PRL suppressed lipolysis in epididymal explants and mature adipocytes in a dose- and time-dependent manner, which was reversed by a Jak2 inhibitor. PRL also inhibited leptin, but not adiponectin, release. We conclude that PRL inhibits lipolysis and leptin release by acting directly on adipocytes via interaction with either of its receptors and activation of a Jak2-dependent signaling pathway(s). This is the first demonstration of substantial effects of PRL on male adipocytes. (c) 2007 Elsevier Inc. All rights reserved.