 Corresponding author: Saheed Olatunbosun Akiode Biotechnology Advanced Research Centre, Sheda Science and Technology Complex, P.M.B 186, Garki, Abuja, Nigeria. Copyright © 2021 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Effect of seasonal changes on the quantity of secondary metabolites from neem and eucalyptus plants in North Central Nigeria Saheed Olatunbosun Akiode 1, * , Adewale Elijah Fadeyi 2 , Anna Asheahe Ibi 1 and Muhammad Bashir Hassan 1 1 Biotechnology Advanced Research Centre, Sheda Science and Technology Complex, P.M.B 186, Garki, Abuja, Nigeria. 2 Chemistry Advanced Research Centre, Sheda Science and Technology Complex, P.M.B 186, Garki, Abuja, Nigeria. World Journal of Biology Pharmacy and Health Sciences, 2021, 07(02), 043–052 Publication history: Received on 29 June 2021; revised on 10 August 2021; accepted on 12 August 2021 Article DOI: https://doi.org/10.30574/wjbphs.2021.7.2.0080 Abstract Traditional healthcare system depends majorly on natural medicinal plants from the environment. These plants produce secondary metabolites which confer on them the various medicinal properties; however, Seasonal fluctuations have impact on their availability and quantity hence their therapeutic efficacy. This study was carried out to evaluate the effect of seasonal changes on the quantity of secondary metabolites from Neem and Eucalyptus plants. Leaves and bark of Neem and Eucalyptus plants from SHESTCO, Federal Capital Territory (FCT), Abuja, Nigeria, were collected during the four quarters of the year 2019. Samples were dried, milled into powder, quantity of secondary metabolites was estimated and antioxidant activity was analysed using standard methods and protocols. Results from this study showed a variation in secondary metabolite compositions in response to seasons. In both plants’ organs, saponin content peaked and crashed during the second and fourth quarters of the year respectively, while alkaloid had the highest content during the fourth quarter of the year. Highest level of tannin was recorded in the leaf and bark of the plants during the third quarter of the year. Antioxidant activity of both plant extracts showed a regular patterned decrease with increasing concentration, with lowest antioxidant activity for both plants’ organs recorded during the third quarter. Findings of the study indicate that the quantity of inherent secondary metabolites in the medicinal plants and their corresponding antioxidant activity varies in specific manners at different times of the year due to seasonal variation. Keywords: Secondary metabolites; Seasonal changes; Antioxidant; Medicinal plants 1. Introduction Medicinal plants are highly valuable to human livelihood. WHO estimated that 60% of the world population and 80% of the population of developing countries rely on traditional medicine, mostly plant drugs, for their primary health care needs [1]. Medicinal plants synthesize a number of biological constituents referred to as secondary metabolites or phytochemicals. These are needed for the plant’s survival and its protection against predators and insects. The secondary metabolites are an extremely diverse group of compounds not only chemically but also in their functions where they are commonly used in agriculture and drug discovery [2-4]. The medicinal value of these plants lies in their bioactive phytochemical or secondary metabolite constituents that produce definite physiological action on the human body [5]. There is a marked variation in secondary metabolite constituents of medicinal plants due to a number of environmental variables such as temperature, altitude, soil type and change of season/rainfall [6-11]. The production of a variety of phytochemicals is the most successful adaptation of plants while developing various physiological mechanisms by