<jats:title>Summary</jats:title><jats:p>Seeds of the plant <jats:italic>Crotalaria retusa</jats:italic> L., containing 4·4% of the pyrrolizidine alkaloid monocrotaline, were fed in a finely ground form to groups of four pigs from 6 weeks of age. Six levels, 0·004, 0·01, 0·02, 0·05, 0·10 and 0·50% by weight of seed in the diet, were fed continuously for up to 19 weeks. In addition a separate regime, designed to mirror an outbreak of poisoning at a local piggery, consisted of feeding 0·10% for 3 weeks followed by 0·05% for 1 week.<jats:p>Exposure to more than 0·01%, whether fed continuously or not, resulted in reduced voluntary feed intake and live-weight gain, and high mortality. At 0·50% a reduction in feed conversion efficiency was also obvious. Levels of 0·004% and 0·01% had no significant effect on the production performance of the pigs.<jats:p>The results of this trial suggest that no more than 0·01% ground <jats:italic>C. retusa</jats:italic> seed should be included in the diets of pigs. When 0·004%–0·01% is included care should be taken as the long-term effect of the subclinical disease produced is not known.