Male weanling pigs (n 32) with a mean initial body weight of 7·5 kg and a mean weaning age of 28 d were used in a 31 d study to investigate the effects of feeding GM (Bt MON810) maize on growth performance, intestinal histology and organ weight and function. At weaning, the pigs were fed a non-GM starter diet during a 6 d acclimatisation period. The pigs were then blocked by weight and litter ancestry and assigned to diets containing 38·9 % GM (Bt MON810) or non-GM isogenic parent line maize for 31 d. Body weight and feed disappearance were recorded on a weekly basis (n 16/treatment), and the pigs (n 10/treatment) were killed on day 31 for the collection of organ, tissue and blood samples. GM maize-fed pigs consumed more feed than the control pigs during the 31 d study (P < 0·05) and were less efficient at converting feed to gain during days 14-30 (P < 0·01). The kidneys of the pigs fed GM maize tended to be heavier than those of control pigs (P = 0·06); however, no histopathological changes or alterations in blood biochemistry were evident. Small intestinal morphology was not different between treatments. However, duodenal villi of GM maize-fed pigs tended to have fewer goblet cells/μm of villus compared with control pigs (P = 0·10). In conclusion, short-term feeding of Bt MON810 maize to weaned pigs resulted in increased feed consumption, less efficient conversion of feed to gain and a decrease in goblet cells/μm of duodenal villus. There was also a tendency for an increase in kidney weight, but this was not associated with changes in histopathology or blood biochemistry. The biological significance of these findings is currently being clarified in long-term exposure studies in pigs.