The aim of the present study was to test the hypothesis that the application of fluoxetine ­ a highly selective serotonin reuptake inhibitor (SSRI) ­ in rats during the suckling period induces changes in testicular development. Groups of newborn male rats were randomly assigned with different doses of fluoxetine 24 hours after birth. Each litter stayed with its respective mother during 21 days. Body weight (BW) was measured daily from the 1st -21st day to calculate daily doses of fluoxetine. 5 mg (T1), 10 mg (T2) 20 mg (T3) or deionized water, were injected intraperitoneally. On the 21st day, animals were heparinized, anesthetized and blood was collected by cardiac puncture to determine by radioimmunoassay the follicle stimulating hormone (FSH) levels. Testis were removed, weighed, and processed for morphometric analysis. Fluoxetine groups presented decreased body and testicular weight when compared with the control group on the 21st day. Our findings show that the manipulation of the serotoninergic system with fluoxetine during the critical period of testicular development alters the Sertoli cell population and all testicular parameters related to this cell.