The purpose of this study was to investigate the acute effect of ethanol on mucosa-associated lymphoid tissue at the level of Peyer's patches and the intestinal lamina propria in female rats and to determine whether this action of ethanol is modulated during the estrous cycle. Adult female rats showing proestrus or diestrus day 1 were treated intraperitoneally (ip) with ethanol (4 g/kg). Untreated and saline-injected rats were used as controls. The animals were sacrificed by decapitation 0.5 h after ethanol administration. Immunoglobulin A (IgA) immunoreactive cells were analyzed by indirect immunohistochemistry using mouse anti-rat IgA and a Dako LSAB+ kit. The number of IgA-immunoreactive cells in Peyer's patches was unaltered by ethanol treatment at both phases of the estrous cycle. However, stereological analysis revealed a significant increase in the number of IgA-immunoreactive cells (p < 0.01) in the intestinal lamina propria following acute ethanol administration at proestrus and on diestrus day 1. The results indicate that the intestinal lamina propria, the effector site of the mucosal immune system, can be affected by a single dose of ethanol at both phases of the estrous cycle.