ABSTRACT One considers a unilamellar liposome filled with an aqueous solution of an osmotic solute. This liposome is introduced into an aqueous medium. Due to the osmosis process, the lipid vesicle swells up to a maximum size, when a transbilayer pore suddenly appears. Part of the internal solution leaks through this pore. The liposome deflates and returns to its initial size. The swelling begins again, and the liposome begins a cyclical evolution. All the processes which contribute to the liposome relaxing and it's coming back to the initial size (pore evolution and internal solution delivery) are described by three differential equations. This system of differential equations describes the evolution of the pulsatory liposome during a cycle and can be integrated using numerical methods. After performing several cycles, the pulsatory liposome stops, and it can be assimilated into a biophysical engine. We calculate the activity time of the pulsatory liposome.