The lipolytic activities of three structurally related mouse hormones, placental lactogen II (mPL-II), GH (mGH), and PRL (mPRL), and human PL (hPL) were investigated. Adipose tissue was obtained from virgin and day 12 and day 16 pregnant mice. Lipolytic activity was assessed by the ability of the hormones to stimulate glycerol release from fat explants in the presence of dexamethasone and by the ability of the hormones to sensitize adipose tissue to the lipolytic stimulus theophylline. In the first experiment, adipose tissue explants were incubated in Krebs-Ringer buffer with 0.0, 0.1, 0.5, 1.0, 5.0, and 10.0 micrograms/ml hormone for 4 h. mGH was lipolytic at a concentration of 0.5 micrograms/ml or greater in adipose tissue from both virgin and pregnant mice. mPRL was lipolytic at a concentration of 5.0 micrograms/ml or greater in adipose tissue from virgin mice. In adipose tissue from pregnant mice mPRL was not lipolytic in day 12 tissue, but it was lipolytic at a concentration of 5.0 micrograms/ml in day 16 tissue. mPL-II and hPL did not stimulate glycerol release in mouse adipose tissue from virgin or pregnant mice. In the second experiment preincubating adipose tissue from virgin mice in the presence of 0.5 or 5.0 micrograms/ml mGH significantly increased the ability of the tissue to respond to theophylline; however, mGH did not induce this response in adipose tissue from pregnant mice, mPRL, mPL-II, and hPL did not increase theophylline-induced lipolysis in adipose tissue from either virgin or pregnant mice. These results indicate that two lipolytic mechanisms are activated in adipose tissue from mice; mGH can activate both mechanisms, whereas mPRL can activate only one.