The effects of mono-(2-ethylhexyl) phthalate (MEHP) and mono-n-pentyl phthalate (MPP) on the ultrastructure of Sertoli cells in primary co-cultures of rat Sertoli and germ cells has been examined and compared with the in vivo effects of di-n-pentyl phthalate (DPP) on Sertoli cells in the rat. MEHP and MPP produced changes in the configuration of the plasma membrane, resulting in long thin cytoplasmic processes or filopodia. This was accompanied by changes in microfilament distribution and an increased density of ribosomes, smooth endoplasmic reticulum and the Golgi complex. Changes were qualitatively similar for both monoesters, but MEHP was effective at lower concentrations than MPP. In vivo, DPP produced extensive convolutions in the basal plasma membranes separating adjacent Sertoli cells and retraction of the lateral processes surrounding germ cells. The similarity of the membrane changes in vivo and in vitro suggests a comparable response by the Sertoli cell in the two systems. Furthermore, the similarity of the in vitro response to the two monoesters suggests a shared mechanism of toxicity.