Galacto-oligosaccharides (GOS) are carbohydrates that are fermented by colonic microbiota. The present study examined effects of a 3-week dietary enrichment with 6 % (w/w) GOS on parameters of energy balance in forty-three male Wistar rats. GOS was tested with two doses of calcium phosphate (30 and 100 mmol/kg), known to differently affect colonic fermentation. After 17 d, isoenergetic test meals were presented and plasma responses of ghrelin, glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) were measured. On day 21 (study termination) epididymal fat pads and caecum were weighed. Additionally, gastrointestinal mucosal samples and proximal colonic contents were analysed for gene expression (ghrelin, proglucagon and PYY) and fermentation metabolites (SCFA and lactate), respectively. GOS reduced energy intake most prominently during the first week, without provoking compensatory overeating later on (average intake reduction: 14 %). The GOS-fed rats showed increased caecal and reduced fat-pad weight and increased gene expression of the satiety-related peptides, PYY (1·7-fold) and proglucagon (3·5-fold). Pre-meal baseline and post-meal plasma levels of PYY, but not of ghrelin or GLP-1, were higher in GOS-fed rats than in control rats. Ca enrichment resulted in higher energy intake (average 4·5 %). GOS diets increased lactic acid levels and slightly reduced butyric acid in proximal colonic contents. Ca abolished the GOS-related elevation of lactic acid, while increasing propionic acid levels, but did not inhibit GOS-related effects on energy intake, fat-pad weight or gene expression. These results indicate that dietary GOS stimulate a number of physiological mechanisms that can reduce energy intake, regardless of the calcium phosphate content of the diet.