Preservice mathematics teachers are subject to multiple influences when they make in-the-moment instructional decisions in classroom settings. Little is known about which influences make a significant contribution to decisions regarding the mathematical content knowledge (MCK) that preservice teachers decide to enact. The process by which mathematics preservice teachers manage multiple influences when making those decisions also requires particular examination. This study investigated six secondary mathematics preservice teachers' content-related decisions and actions in ten lower secondary algebra lessons using a combination of lesson observations and stimulated recall interviews. The study uncovered five influences, each comprising a number of influencing elements, that led the preservice teachers to intentionally enact or alternatively to intentionally withhold certain aspects of their MCK during algebra lessons. Combinations of harmonious influences were regularly accommodated by preservice teachers, while other combinations of influences competed for the preservice teachers' attention at different points in their lesson. The findings reveal that simply possessing content knowledge does not mean that preservice teachers are prepared to impart that knowledge to their students in the classroom context. The multi-faceted nature of content-related decision-making for preservice teachers suggests that content knowledge development should not be attended to in isolation within teacher education programs but should be located in teaching contexts wherever possible.