Light adaptation in photoreceptor cells is a prerequisite for normal vision in the vastly variable light intensities found in natural environments. We investigated in vivo how the feedback from large monopolar cells (LMCs) onto photoreceptors modulates this process by recording intracellular voltage responses of photoreceptors in the mutants Vam and ort⁵. In Vam flies, LMCs 1 and 2 degenerate and ort⁵ flies are a null mutant for the histamine receptor in LMCs. By repeating saturating light pulses of 4 LEDs, we show that both Vam and ort⁵ have faster response dynamics (A, inset). From trace 2 onwards, Vam responses are significantly smaller than wild type (Canton S) while ort⁵ showed no difference to wild type (A, Trace 2, p = 0.00036 & p = 0.65 respectively, t-test), leaving Vam with a smaller voltage range for extra light adaptation. All flies showed a further reduction in amplitude recorded in response to 100 LEDs, highlighting a general lateral inhibitory field effect (B) which was proportionally larger in Vam photoreceptors (C). Both mutants displayed slower repolarisation in response to a long light pulse (D & E, arrows). Vam also lacked a sustained response during the first light pulse (D, asterisk), a phenotype not previously reported in mutants with defective or absent LMCs. Our results provide further evidence that the LMC feedback actively modulates photoreceptor voltage responses¹. Without such feedback, the photoreceptors light adaptation is defective, resulting in suboptimal use of response range for coding of visual information. Future work includes ablation of LMCs with ricin using the UAS-Gal4 system. This approach will clarify whether the missing sustained response during the first prolonged light pulse in Vam is solely attributable to a lack of LMCs 1 and 2.