Although studies have shown that prolactin (Prl) and metoclopramide (Mcp) administration restores the depressed cell-mediated immune functions after hemorrhage, the underlying mechanism responsible for the immunostimulatory effects of Mcp remains unknown. We hypothesized that Mcp improves immune responses by upregulating the secretion of Prl. To test this hypothesis, male C3H/HeN mice were subjected to sham operation or laparotomy (i.e., soft tissue trauma) and hemorrhagic shock (Hem; 35 ± 5 mmHg for 90 min) and then resuscitated. Plasma Prl levels were determined 30 min after Mcp (1 μg/g body wt sc at end of Hem) or vehicle (Veh) treatment in sham and Hem mice. The results indicate that plasma Prl levels increased significantly in Mcp-treated mice (sham-Veh 249.9 ± 5.3, Hem-Veh 229.9 ± 7.6, Hem-Mcp 596.9 ± 73.1 ng/ml, one-way ANOVA, P < 0.05 vs. Veh). To determine whether Mcp produces its salutary effects directly or indirectly via increased Prl secretion, splenocyte proliferation and splenocyte interleukin (IL)-2 and IL- 3 release from untreated sham or Hem mice were determined in the presence of increasing concentrations of mouse Prl or Mcp. The addition of Mcp had no effect on splenocyte immune functions in vitro. However, the addition of Prl restored the hemorrhage-induced depressed splenocyte proliferation as well as splenocyte IL-2 and IL-3 release in vitro in a dose-dependent manner. Thus the beneficial effects of Mcp on immune functions after Hem appear to be mediated by Prl. Because Mcp increases plasma levels of the immunoenhancing hormone Prl, this agent should be considered a useful adjunct for the treatment of immunodepression in trauma victims.