Experiments to study the effect of extrinsic factors on the copulation of normal adult Nippostrongylus brasiliensis are described. Worms copulated when in contact with the rat small intestine, whether it was ligatured, maintained inside or outside the peritoneal cavity, slit open or its lumen obstructed, provided that the blood supply remained intact and the temperature was within a few degrees of normal. Copulation did not occur in the colon, pleural cavity, peritoneal cavity, pregnant or non-pregnant uterus of the live rat or under a variety of in vitro conditions. These results indicate the importance of extrinsic factors for successful copulation since all the worms used in the experiments were intrinsically ready for immediate copulation in their normal environment. Worms observed during in vitro experiments were more vigorous and erratic in their movements than worms in their normal environment. The possible significance of this observation is discussed both in the context of in vitro reproduction experiments and in vitro experiments to study the physiology and biochemistry of the worm.