Studying the gastrointestinal motor effects of iron compounds may help to elucidate the mechanism originating the gastrointestinal side effects frequently reported during martial therapy. The aims of the present study were: (1) to examine the gastrointestinal motor effects of ferrous sulfate (reference compound) and (2) to compare its effects with those of two iron succinyl-protein complexes (ITF 1096 and ITF 282, an iron-albumin and iron-casein complex, respectively). In 6 fasting, conscious dogs, fitted with 8 bipolar electrodes and 3 strain-gauge force transducers along the gastrointestinal tract, gastrointestinal motor activity was recorded. Ferrous sulfate and iron succinyl-protein complexes were administered by an orogastric tube at two dose levels: the lower and higher dose levels were approximately equivalent to 10 and 30 mg/kg as Fe, respectively. In control experiments, 154 mM NaCl, ITF 211 (succinylated albumin) and ITF 297 (succinylated casein) were used. Administration of 154 mM NaCl did not affect gastrointestinal motility nor did it disrupt migrating motor complex (MMC) cycling. ITF 1096 and ITF 282, only at the higher dose, lengthened the MMC period and increased intestinal, but not gastric spike activity. The effects of ITF 211 and ITF 297 were similar to those of ITF 1096 and ITF 282. Ferrous sulfate, at the lower dose, lengthened the MMC period; the higher dose disrupted MMC cycling and induced intense, irregular spike bursts in the stomach and in the small bowel, accompanied, in 3 out of 6 dogs, by a prolonged tonic contraction of the upper small bowel. Four out of 6 dogs vomited after the higher dose of ferrous sulfate. No vomiting was observed with any of the other treatments. We conclude that ITF 1096 and ITF 282 have a markedly better gastrointestinal tolerability than ferrous sulfate.