A prospective study was performed to compare the contractility of the gall bladder ultrasonically after ingestion of a 65 g Mars bar or 60 ml of Calogen. Using a repeated measure design, 27 healthy subjects were studied fasting and at 10 min intervals after ingestion of the fatty meal. Results show that there is a significant difference between Mars bar and Calogen (ANOVA, P < or = 0.01), with Calogen producing greater gall bladder contraction overall, and that the ejection fraction is greatest at 40 min (t = 2.23, P < or = 0.01). It is suggested that 60 ml of Calogen is used as the standard fatty meal to assess gall bladder contractility and that the patient is rescanned 40 min post fatty meal.