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BACKGROUND: Arginine vasopressin has been shown to enhance learning in experimental animal models. OBJECTIVE: To determine whether Guipi decoction enhances memory and learning by increasing arginine vasopressin levels, and to verify the influence of Guipi decoction on arginine vasopressin protein and gene expression in the hippocampal CA1 region, prefrontal lobe cortex, and ventral nucleus of hypothalamus in rats with spleen deficiency. DESIGN, TIME AND SETTING: The randomized, neuropharmacological, control study was performed in the College of Basic Medical Sciences, Beijing University of Chinese Medicine between March 2002 and March 2005. MATERIALS: Sixty, healthy, male, Wistar rats were used to establish spleen deficiency models according to the traditional Chinese medicine principle of bitter drugs for purgation, improper diet, and overstrain. Arginine vasopressin-1 polyclonal anti-rabbit antibody immunohistochemistry kit and arginine vasopressin in situ hybridization kit were provided by Department of Neuroanatomy in Shanghai Second Military Medical University of Chinese PLA. METHODS: Sixty rats were divided into five groups at random: normal control (n = 11), model (n = 13), Guipi decoction (n = 12), recipe control A (n = 12), and recipe control B groups (n = 12). Rats in the latter four groups received 7.5 g/kg of the drugs by intragastric administration each morning, which comprised Dahuang, Houpu, and Zhishi, prepared at a ratio of 2:1:1. The rats were fasted every other day, but were allowed free access to water at all times. The rats were forced to swim in 25 ℃ water until fatigued. Rats in the Guipi decoction and two recipe control groups were intragastrically administered 7.5 g/kg Guipi decoction, Chaihu Shugan powder, and Tianwang Buxin pellets, respectively, each afternoon. Rats in the normal group were intragastrically administered the same amount of normal saline. All rats were treated for 6 weeks. MAIN OUTCOME MEASURES: At 6 weeks after drug administration, rat brain tissues were harvested. Arginine vasopressin-positive protein products and arginine vasopressin mRNA-positive products in the ventral nucleus of hypothalamus, hippocampal CA1 region, and cortex of prefrontal lobe were determined using immunohistochemistry and in situ hybridization, respectively. Image collection and management were performed by means of an Olympus microscope and color image analyzer system. Average absorbencies were measured. RESULTS: Qualitative analysis: in the normal control group, the arginine vasopressin protein- and mRNA-positive cells were round and oval. The immunoreactive products were strongly positive, with a uniform coloration. In the model group, the arginine vasopressin protein- and mRNA-positive cells exhibited a shortening or disappearance of neuronal processes, as well as cytoplasm that exhibited chromatic non-uniformity. After rats were administrated Guipi decoction, the arginine vasopressin protein- and mRNA-positive cells exhibited rough and long neuronal, and the intensity of cytoplasmic coloration was also enhanced. Quantitative analysis: in the model group, arginine vasopressin protein and mRNA expression was significantly less in the hippocampal CA1 region, cortex of prefrontal lobe, and ventral nucleus of hypothalamus, compared with the normal group. Compared with the model group, the above-mentioned indices were remarkably increased in the Guipi decoction group (P < 0.01), and were more similar to normal levels. Rats treated with Chaihu Shugan powder or Tianwang Buxin pellets displayed increased arginine vasopressin protein and mRNA expression, but the therapeutic effect was inferior to Guipi decoction. CONCLUSION: Qualitative and quantitative analyses demonstrated that the Guipi decoction enhanced arginine vasopressin protein and gene expression in the hippocampal CA1 region, prefrontal lobe cortex, and ventral nucleus of hypothalamus in rats with spleen deficiency.