Background: The study aimed at assessing total alkaloids content (TAC), total flavonoids content (TFC), total phenolic content (TPC), antioxidant activity (AA) and predominant individual phenolic compounds in methanol extracts of M. oleifera, S.singueana, M.azedarach leaves and stem barks of L. discolor collected during different seasons of the year. Materials and Methods: TPC, TFC and TAC were analyzed using the Folin Ciocalteau assay, aluminum chloride assay and bromocresol green assay, respectively. 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) techniques were used to analyse AA. Phenolic compounds; Vanillic acid, (+)-catechin, quercetin, rutin, sinapic acid, 2,4-hydroxybenzoic acid and ferulic acid were analyzed using high performance liquid chromatography diode array detector (HPLC - DAD). Results: Barks of L. discolour exhibited high levels of TPC, TFC, TAC and AA during hot-dry season (summer) as compared to S. singueana, M. oleifera and M. azedarach leaves which showed significantly (p< 0.05) higher levels in hot-wet season (rainy). Levels of TPC, TFC and TAC were significantly (p< 0.05) influenced by seasonal variations and correlated with levels of AA. Profiles of all phenolic compounds analyzed were not similar in terms of chemical composition and concentration during different seasons. Sinapic acid and 2,4-hydroxybenzoic acid were predominant in S. singueana while sinapic acid and ferulic acid were predominant in M. oleifera. Vanillic acid and 2,4-hydroxybenzoic acid were predominant in L. discolor and M. azedarach respectively. Conclusion: The phytochemical constituents in medicinal plants are subject to seasonal variations which in turn influence the optimum season of harvesting.