The relative activities of synthetic prostaglandins of the A, B, E and F classes in causing expulsion of N. brasiliensis worms from the small intestine of rats was compared by intraduodenal injection on day 6 of a primary infection. This showed that prostaglandins E1 and E2 were highly effective. The prostaglandins of the A and B classes were less effective than those of the E class. Injection of PGF1α or F2α caused a modest reduction in the worm count when compared with infection controls, but this was not statistically significant. In a parallel study, the capacity of the biogenic amines (histamine and 5-hydroxytryptamine) to cause worm expulsion was also tested by intraduodenal injection on day 6 of a primary infection. Neither amine had any activity.The results are discussed in context with current knowledge of the mechanism of worm expulsion, and it is concluded that prostaglandins (particularly of the E class) are more likely to be endogenous mediators of expulsion of N. brasiliensis in the rat than the biogenic amines.