The development and behavior of the preparasitic stages of Nippostrongylus brasiliensis, and the development of the parasitic stages in the laboratory rat are reviewed. The life cycle can be summarized as follows: Most of the eggs hatch 20 to 24 hours after passing out of the host in the feces, and larval development of the infective stage requires an additional 3 to 4 days. Infective larvae enter the host by penetration of the skin and about 15 hours later the majority of them have migrated to the lungs. In the lungs, the larvae feed, grow and undergo the third molt, and by about the 59th hour of infection most of them have migrated via the respiratory tree esophagus, and stomach to the small intestine. After the fourth molt the worms become sexually mature and grow to their final size. The prepatent period commonly is 6 days and the patent period may last from a few weeks to several months. The maximum population of adult worms occurs toward the end of the first week of infection after which there is loss of worms. The majority of the worms are lost during the second and third week of infection and the magnitude of worm loss at that time is related to the size of the initial adult worm population. N. brasiliensis is pathogenic and produces deleterious effects in rats during the passage of larvae through the lungs, and during the residence of the adults in the gut. The infected small intestine has an increased water and tissue content, and there are changes in the concentrations of fat and electroytes in the jejunal tissues. The rate of d-glucose absorption from the infected jejunum is greatly reduced, and the rate of protein digestion also is depressed. Factors that have been found by various investigators to influence the development of N. brasiliensis in rats include host breed, age, and diet, parasite strain, age of infective larvae, route of infection, and the techniques for handling infective larvae. The importance of these and other factors are discussed.