This study examined the effect of chronic cocaine exposure on selected immune parameters in pregnant rats. Cocaine hydrochloride, 60 mg/kg, was administered by i.p. injection as a divided daily dose on gestation days 8 to 19. This cocaine treatment regimen did not result in any change in maternal body weight, spleen and thymus body weight ratios or lymphocyte recovery from these organs. Cocaine treatment had no effect on the plasma levels of prolactin, growth hormone and insulin-like growth factor-1; hormones with immunoregulatory potential. In contrast, the plasma immunoglobulin G concentration in cocaine-treated animals was 48% higher (P < .05) than in control animals. Spleen lymphocytes and thymocytes were isolated and evaluated for their proliferative responses in vitro to a panel of T and B cell mitogens. Lymphocytes from cocaine-treated animals showed no significant differences in proliferative responses to concanavalin A (conA), phytohemagglutinin, pokeweed mitogen, interleukin-2 or lipopolysaccharide. The ability of conA-stimulated spleen lymphocytes to synthesize and secrete prolactin-immunoreactive proteins was further assessed by Western immunoblotting. We found that conA-stimulated spleen lymphocytes from cocaine-treated animals showed significantly decreased levels of intracellular and secreted 44,000-mw prolactin-immunoreactive proteins. In contrast, conA-stimulated spleen lymphocytes from control and cocaine-treated groups secreted equivalent amounts of the cytokine interleukin-2. In conclusion, chronic administration of cocaine to female rats during pregnancy significantly altered serum immunoglobulin G levels and lymphocyte production of prolactin-immunoreactive proteins in the absence of changes in lymphocyte proliferation in response to mitogens.