Sporozoites of Plasmodium berghei berghei (NK-65 strain) were inoculated intravenously into four-week-old CFN rats. Liver biopsies were taken at intervals and the density of exo-erythrocytic forms per mm3 of liver were estimated. Results demonstrated asynchronous maturation and the gradual disappearance of exo-erythrocytic forms during the period 48 to 72 hours after infection.