The isotope dilution technique was used to evaluate calcium true digestibility and calcium balance in finishing lambs fed different calcium sources. The experiment was carried out at the Animal Nutrition Laboratory of the Centre for Nuclear Energy in Agriculture, University of Sao Paulo (CENA/USP). Twenty Santa Ines non-castrated male lambs, aged 7-8 months were used. Treatments consisted of a basal diet (corn, soybean meal, hydrolysed sugarcane bagasse, urea, monoammonium phosphate (MAP) and mineral mixture) supplemented with five calcium sources: citrus pulp (diet CTP), Lucerne hay (diet LUC), limestone (diet LIM), oyster shell meal (diet OSM) and dicalcium phosphate (diet DCP). After 21 days for diet adaptation, the lambs were allocated into metabolic cages and each animal was injected intravenously with 7.7 MBq of 45 Ca, as calcium chloride in aqueous solution. Blood samples were collected at 5 min, 1, 2, 4, 6, 24, 48, 72, 96, 120 and 144 h after injection. Faeces and urine samples were collected at each 24 h after injection for 7 days. The radioactivity of plasma, faeces and urine samples were determined by a liquid scintillation method. Comparison of the means (Duncan test) and regression analysis were carried out, and for statistical analysis the experiment was considered a randomised design with five treatments and four replicates. Animals fed either diet CTP (98\%) or diet LUC (100\%) had higher calcium faecal output than those fed diet LIM (58\%). The percentage of endogenous loss varied from I I to 20\%, and was not influenced by either dry matter or calcium intakes. Calcium absorption and true digestibility was highest for diet LIM. Calcium retention for animals fed diet LIM was higher than those fed either or diets CTP and LUC. Calcium availability and metabolism was affected by dry matter and calcium intake and by the chemical form of calcium in the diet. It is recommended that citrus pulp and Lucerne hay should not be used as the sole calcium source in a diet for finishing lamb. (C) 2006 Elsevier B.V. All rights reserved.