The gonadotrophin-releasing hormone (GnRH) neurons represent the key output cells of the neuronal network controlling the hypothalamic pituitary-gonadal axis and thus fertility in mammalian species. The patterns of neuronal activity by which GnRH neurons generate pulses of GnRH and thus luteinizing hormone (LH) are unknown. The ability to manipulate GnRH neurons in a selective manner in vivo would facilitate our understanding of the patterns of activity required to generate pulses of LH. We used a Cre-dependant adenoassociated virus (AAV) injected into the median eminence of transgenic GnRH-Cre mice to target channelrhodopsins (ChR2) specifically to the hypophysiotropic GnRH neurons. Immunofluorescence studies showed that ≈93% of all ChR2-expressing cells expressed GnRH. We first characterized the ability of ChR2 to modulate GnRH neuron excitability in vitro and showed that blue light stimulation using 5ms pulses at 10Hz and 30Hz for up to 5min induced action potential firing with a response fidelity of 85% and 72% respectively. We then stimulated the GnRH neurons in vivo in AAV injected GnRH-Cre female ovariectomized mice by placing an optic fiber in the rostral preoptic area and applied different patterns of activation in order to elicit LH secretion. Activation of GnRH neurons for 5min at 1Hz and 5Hz was found to have no effect on LH secretion. Stimulation at 10Hz (p=0.0024) and 30Hz (p=0.0008) produced significant pulse-like increases in LH secretion. Synchronized burst firing has been suggested to underlie GnRH pulsatility. As such, GnRH neurons were synchronously activated for 5min to fire in bursts using the burst patterning previously detected for GnRH neurons in vivo. This was found to have no effect on LH secretion. Together, these studies highlight the patterns of GnRH neuron electrical activity required to generate pulsatile LH secretion.