RECENT investigation of the light emitted by gallium arsenide lasers1–3 when current pulses of widths in the tens of nanoseconds range are passed through the p–n junction have led to the observation in this laboratory that a certain charge must flow into the junction before laser action commences. For a given amplitude of the current pulse, there is a corresponding pulse width which will just induce laser action, the laser action commencing on the falling edge of the pulse. The converse is also true. As pulse-width is decreased, the pulse-amplitude must be increased if laser action is to be achieved. Statements that the laser threshold current density under pulse conditions is x amp/cm2 therefore become meaningless unless the pulse widths at which the measurements were made are also stated. Long time constants have also been observed in the light pulse emitted from the forward biased p–n junction.