In order to better understand how the coccidiocide monensin affects T. gondii, we analyzed the effect of monensin on gene expression of intracellular parasites through microarrays. We compared RNA collected either from RH strain tachyzoites 24 hrs after invasion of human foreskin fibroblasts, or 24 hrs after invasion followed by 24 hrs exposure to 2.5ng monensin/ml in tissue culture medium.