Circadian rhythms in conditioned threat extinction emerge from a tissue-level circadian timekeeper, or local clock, in the ventromedial prefrontal cortex (vmPFC). Yet it remains unclear how this local clock contributes to extinction-dependent adaptations. Here we used single-unit and local field potential analyses to interrogate neural activity in the male rat vmPFC during repeated extinction sessions at different times of day. In association with superior recall of a remote extinction memory during the circadian active phase, vmPFC putative principal neurons exhibited phasic firing that was amplified for cue presentations and diminished at transitions in freezing behavior. Coupling of vmPFC gamma amplitude to the phase of low-frequency oscillations was greater during freezing than mobility, and this difference was augmented during the active phase, highlighting a time-of-day dependence in the organization of freezing- versus mobility-associated cell assemblies. Additionally, a greater proportion of vmPFC neurons were phase-locked to low-frequency oscillations during the active phase, consistent with heightened neural excitability at this time of day. Our results suggest that daily fluctuations in vmPFC excitability precipitate enhanced neural recruitment into extinction-based cell assemblies during the active phase, providing a potential mechanism by which the vmPFC local clock modulates circuit and behavioral plasticity during conditioned threat extinction.Significance StatementConditioned threat extinction is a learning and memory process by which exposure to danger-predictive cues prompts a reduction in defensive behavior. The recall of extinction memories exhibits a robust circadian rhythm, such that recall is stronger during the circadian active phase. However, the mechanisms underlying this circadian rhythm remain unclear. Here we examined neural activity within the rat ventromedial prefrontal cortex, a brain region supporting extinction, in repeated extinction sessions at two times of day. Multiple aspects of single-cell and population activity exhibited extinction-dependent time-of-day differences. Our findings suggest that heightened neural excitability during the active phase promotes the recruitment of neurons into extinction-dependent cell assemblies, offering a mechanism to explain enhanced extinction recall at this time of day.