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y-frequency oscillations (30-120 Hz) in cortical networks influence neuronal encoding and information transfer, and are disrupted in multiple brain disorders. While synaptic inhibition is important for synchronization across the y-frequency range, the role of distinct interneuronal subtypes in slow (<60Hz) and fast y states remains unclear. Here, we used optogenetics to examine the involvement of parvalbumin-expressing (PV+) and somatostatin-expressing (SST+) interneurons in y oscillations in the mouse hippocampal CA3 ex vivo, using animals of either sex. Disrupting either PV+ or SST+ interneuron activity, via either photoinhibition or photoexcitation, led to a decrease in the power of cholinergically induced slow y oscillations. Furthermore, photoexcitation of SST 1 interneurons induced fast y oscillations, which depended on both synaptic excitation and inhibition. Our findings support a critical role for both PV+ and SST+ interneurons in slow hippocampal y oscillations, and further suggest that intense activation of SST+ interneurons can enable the CA3 circuit to generate fast y oscillations. [ABSTRACT FROM AUTHOR]