Experimental infection by F. hepatica was performed on rats. Early recruitment of the peritoneal cell population was observed and revealed transient parasite-killing activity, preceded and followed by a state of total unresponsiveness. The activation peaked at seven days post-infection (dpi) and was characterised by a massive peritoneal cell recruitment, a strong superoxide anion and nitric oxide (NO) production, that were coincident with the fasciolicide activity of these cells, as monitored by an in vitro decrease of juvenile fluke viability in a conditioned medium. The addition of L-NG-monomethyl arginine (LNMMA) to cell cultures abrogated both fasciolicide activity and NO production. Parasites started to die when NO production exceeded 25 microM and all juvenile flukes were killed by a 90 microM NO exposition (Lethal Dose 50 between 45.8 and 50.3 microM, 95% fiducial limits). However, when rat peritoneal cells were cultured in the presence of either infected or control rat serum, juvenile flukes were much more resistant to the oxidative burst, despite a massive attachment of rat peritoneal cells to the parasite tegument. These data suggest that a transient control of fasciolosis may take place in the peritoneum following the parasite intrusion but that the parasite efficiently scavenges the host cellular response to avoid destruction.