eng Rovedar Journal of Veterinary Physiology and Pathology 2821-0328 2025-12-29 4 4 64 68 10.58803/jvpp.v4i4.76 78 Innocent Jonah Gosomji 0 https://orcid.org/0000-0002-8727-3217 Oludayo Akinsola 1 https://orcid.org/0000-0003-3981-5672 Abdullahi Baso 2 https://orcid.org/0000-0001-6731-853X Nura Ahmed 3 https://orcid.org/0009-0002-0137-3987 Jamiu Oyewole Omirinde 0 https://orcid.org/0000-0002-2952-9376 Idris Ayodeji Azeez 0 https://orcid.org/0000-0001-5858-1395 Naanman James Plang 0 https://orcid.org/0000-0001-5695-6325 Nenchini Bala Koplamma 4 Sunday Akau Hena 0 https://orcid.org/0000-0002-5231-4947 Department of Veterinary Anatomy, Faculty of Veterinary Medicine, University of Jos, Nigeria Department of Theriogenology and Production, Faculty of Veterinary Medicine, University of Jos, Nigeria Department of Veterinary Anatomy, Bayero University, Kano, Nigeria Department of Veterinary Anatomy, Ahmadu Bello University, Zaria, Nigeria Department of Biology and Forensic Science, Admiralty University of Nigeria, Ibusa, Nigeria Introduction: Guinea fowls have sophisticated seasonal mechanisms that make them an excellent model for studying reproductive activities. The present study aimed to investigate the impact of exogenous melatonin and artificial light on testicular morphometry in sexually mature guinea fowls. Materials and methods: A total of 65 mature male guinea fowls, with an average weight of 1-2 kg, were randomly selected for the present study. Five guinea fowls were utilized for the primary evaluation study on day 0. Sixty guinea fowls were divided into six groups with 10 guinea fowls in each group. The first group (SD) was exposed to 8 hours of light and 16 hours of darkness (8L:16D). The second group (SD-Mel) was exposed to 8L:16D with an additional melatonin injection. The third group was the control group (CTL) with 12L:12D. The fourth group was maintained on CTL with an additional injection of melatonin (CTL-Mel). The fifth group (LD) was exposed to 16L:8D, and the sixth group was exposed to 16L:8D with an injection of melatonin (LD-Mel).  Melatonin was administered intramuscularly at 1 mg/kg body weight 2 hours before the end of each light cycle. The entire study was conducted in 60 days. Body weight (BW), testicular weight, and gonado-somatic index (GSI) were recorded at days 0, 30, and 60 of the study. Results: There was a consistent increase in the BW, testicular weight, and GSI from days 0 to 60 in all groups. The results of BW, testis weight, and GSI were statistically significant on days 0, 30, and 60 across all groups. The testicular weight was higher in SD and LD-Mel, while the lowest weight was in SD-Mel. The GSI was higher in SD and LD-Mel, while the lowest was in SD-Mel. In the LD-Mel and LD groups, there was a strong positive correlation between testes and GSI. Conclusion: Melatonin exposure, particularly under long-day conditions, enabled guinea fowl to improve reproductive potential by day 30 of light treatment. https://jvpp.rovedar.com/index.php/JVPP/article/view/76 Guinea fowl Melatonin Morphometry Photoperiod Testis