The use constant values to represent the loss of energy due to mobilization of fat and protein as considered by INRA (1989), AFRC (1993) and Morand-Fehr and Hervieu (1999) does not reflect in a reliable dynamic mobilization. It should be considered that most data used for that inference were based on short-term calorimetric studies. Thus, it is essential to quantitatively know low body energy reserves are transformed into milk energy, and low energy mobilized and consumed concur for energy balance in the lactation course. This study aimed to measure changes of body fat and protein masses, and also quantify mass-changes of visceral organs of Swiss Alpine does throughout the first 8 weeks of lactation. Fifty-one multiparous does were allotted in a completely randomized design with 6 replications and 8 treatments (weeks of lactation). Animals were slaughtered sequentially to measure the mass of fat and protein retained in the body, as well as the weight of the visceral organs. The data were analyzed by using the MIXED procedure of SAS, in which 2 covariance structures (homogeneous and heterogeneous variances) and the likelihood of linear, quadratic, and cubic models were evaluated. The empty body weight (EBW) decreased (P < 0.05) over the 8 weeks of lactation (from 56.8 to 35.6 kg) and carcass fat also decreased (P < 0.05) from 5.16 to 2.10 kg and from 9.1 to 5.9% of EBW. Body fat reduced from 24.13 to 13.51%, as well as protein (from 14.18 to 14.75%), mostly because of the mobilization of noncarcass fat (from 28.58 to 15.73%) and protein (from 12.42 to 13.20%). The visceral fat mass was mobilized with greater intensity from the third to the eighth week of lactation, i.e., 4.33 kg reduced to 1.40 kg in the period or 7.57% reduced to 3.01% of EBW. There was no effect (P > 0.05) on liver mass, rumen, omasum, abomasum, small intestine, and large intestine during the first 8 weeks of lactation, but there was a decrease (P < 0.05) in the uterus (from 1.63 to 0.14 kg) and mammary gland masses (from 2.84 to 1.48 kg). In conclusion, Swiss Alpine does in early lactation mobilized energy reserves not only from carcass and internal fat but also from visceral organs. Funded by FAPERJ, CAPES and CNPq.