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Gonadotropin-releasing hormone (GnRH) analogues are used for the treatment of breast, prostate, and ovarian cancer. These analogues exert their antitumor effects indirectly by inhibiting the pituitary-gonadal axis, as well as by direct inhibition of proliferation of human breast cancer cells. However, the molecular mechanisms mediating these direct antiproliferative effects are not fully understood. We found that normal and malignant human breast tissue express two forms of the neuropeptide GnRH. Quantitative reverse transcription-PCR shows that mRNA encoding the GnRH-I and GnRH-II peptides are overexpressed in cancerous versus normal tissues obtained from the same patients. To elucidate the function of these peptides in breast cancer cells, we used the atlas human cDNA expression arrays technology and studied the differentially regulated genes after GnRH treatment of MCF-7 cells. We found that a wide range of GnRH-I or GnRH-II concentrations (0.1-10 nM) inhibit the expression of mRNA encoding the 60S acidic ribosomal phosphoproteins, P1 and P2. These results were confirmed by quantitative reverse transcription-PCR, as well as Western blot analysis and immunofluorescence staining. The P1 and P2 proteins interact with elongation factors EF1 and EF2, and the level of their phosphorylation is one of the regulatory mechanisms for the overall rate of protein elongation. Thus, reduced expression of P1 and P2 proteins can affect the rate of protein translation, thereby decreasing proliferation rate of cells. Our studies may therefore suggest a putative mechanism for the direct antiproliferative effect of GnRH in breast cancer cells.