The purpose of this paper was to find out the evolution of the main haematological parameters in Hy-Line var. Brown hens raised in industrial system, during a natural moult process. The process of spontaneous moult started towards the end of the laying cycle (63 weeks of age) and lasted about seven weeks. Two groups were set up for monitoring: a control group of hens that did not show signs of moult and an experimental group of spontaneously moulted hens. Blood was sampled weekly, from the moment of observing the onset of moult, to the complete re-dressing with feathers, respectively a period of seven weeks. Haematological parameters showed a specific dynamic during this process. This dynamic is dominated by a decrease in red blood cells, packed cell volume and haemoglobin (P<0.05), on the one hand, and a decrease in the white cell number and in the percentage of lymphocytes (P<0.05) on the other. These changes take place predominantly in the first three weeks of the moult period. In the next period of the process, the modified haematological parameters return to the values comparable to those of the control (P > 0.05).