In the present study we investigated the influence of Walker 256 tumor growth on the modification of placental morphology and on fetal development in young and adult pregnant rats. After mating, female rats were divided into six groups: young control pregnant (Y), young pregnant with tumor (Yw), young pregnant injected with ascitic fluid (Ya), adult control pregnant (A), adult pregnant with tumor (Aw), and adult pregnant injected with ascitic fluid (Aa). Rats from tumor-bearing groups (Yw and Aw) were injected with 2.5 x 10(6) viable tumor cells into the right flank. Rats from Ya and Aa groups received daily inoculations of ascitic fluid (2.0 ml, i.p.) obtained from tumor-bearing rats without tumor cells. After 21 days, all animals were killed and the placentas were weighed and fixed with paraformaldehyde for histological analysis. Compared with control groups (Y and A), both tumor-bearing groups (Yw and Aw) presented the following changes: i) hemorrhage in the decidua and in the trophoblast giant cell layer; ii) disarrangement of the spongy zone, iii) restricted delimitation of the maternal and fetal blood vessels in the placental labyrinth; iv) hemorrhage and edema in the placental labyrinth. Similar results were observed in the placenta of groups injected with ascitic fluid (Ya and Aa). These results indicate that tumor development during pregnancy can have deleterious effects on placenta and fetus. These observations extend our previous data of extensive fetal reabsorption in both pregnant tumor-bearing and ascitic fluid-injected animals. These changes in placental morphology may be related to the synthesis and release of some factors by the tumor and the host cells, which could act directly or indirectly on placental tissue.11359-66