The need to examine the problem of high legislative turnover that has become a recurring feature of national and state parliaments in Nigeria necessitates this study. For instance, the 2014 National Institute of Legislative Studies' (NILS) Report indicates that Nigeria's National Parliament has the highest turnover rate of legislators (70 per cent) in the world. The scenario is even worse in State Assemblies where more than 80 per cent of members are greenhorns at the beginning of every legislative session. To know the implications of high membership turnover on legislative performance in Nigeria's parliament, this study examines membership turnover and legislative performance in two of Nigerian States, Kogi and Osun between 1999 and 2015. The objectives of the study are to i) assess the rate of membership turnover in Kogi and Osun State Assemblies; ii) investigate the causes of high membership turnover in Kogi and Osun State Assemblies; iii) examine the effects of high membership turnover on law-making, oversight and constituency services in Kogi and Osun States; and iv) compare the findings in Kogi and Osun State. The study employs a combination of quantitative and qualitative methods, while data are gathered from primary and secondary sources. Interviews are conducted with past and present legislators, legislative staff and other key players in legislative activities from Kogi and Osun State assemblies. Secondary data are generated from official records of Kogi and Osun State Assemblies, National Assembly, NILS, textbooks, journals, internet materials etc. Content analysis and descriptive statistics are used to analyze data. The study employs a combination of three theories- elite, system, and structural functionalism for its analysis. Findings reveal that turnover rates in Kogi and Osun State assemblies stand above the highest 'normal' limit of 40 per cent (88 per cent and 85.6 per cent, respectively). The causes of high turnover rates of members in Kogi and Osun States are attributed to: politics of zoning and power sharing; lucrative nature of politics; electoral defeat; electoral malpractices; legislators' performances; godfatherism, and party politics. The study also reveals the negative effects of high turnover rates of members on law-making, oversight and constituency services in Kogi and Osun States. A comparison of findings in Kogi and Osun show that the high turnover rate affects Kogi State Assembly more than Osun State Assembly on law-making function. Besides, the study discovers that the challenges of Kogi and Osun State legislatures go beyond the issue of high turnover rates of members. Other challenges faced by the legislatures of the two states are identified in the study. The study recommends a systematic training and re-training programmes on capacity building of fresh lawmakers; financial autonomy; functional State Assembly Service Commission; and adequate infrastructural facilities, among others. To reduce the rate at which politicians rush for legislative seats and to bring down turnover rate of legislators, downward review of legislators' salary is considered necessary. The electoral system should permit independent candidates to contest elections; the study also recommends a paradigm shift from politics of zoning to that of performance.