The correspondent mass accretion rate values for SS Cygni are deduced from the He II 1640 A emission line equivalent width values in all good available IUE low resolution spectra, using the optically thick boundary-layer model developed by Patterson and Raymond (1985). In the rising and declining phase of an outburst, the mass accretion rate value is consistent with the commonly accepted theoretical models on the quiescence-outburst optical behavior of dwarf novae, which suppose that luminosity depends only on the rate at which matter is accreted. The mass accretion rate in outburst is less than that in quiescence, suggesting that the accretion is not the only energy release mechanism responsible for the higher luminosity. The results of studies to search for alternative and/or additive mechanisms in producing the highest luminosities associated with the outbursts are discussed.