This study examined the relationship between monetary policy instruments and total investment spending in Nigeria, with particular focus on the effects of money supply and the monetary policy rate. The analysis was based on annual time series data covering the period from 1978 to 2023 and employed the Ordinary Least Squares (OLS) estimation technique. The empirical results revealed that money supply had a positive and statistically significant effect on aggregate investment expenditure, indicating that liquidity expansion supported investment activities within the economy. In contrast, the monetary policy rate exerted a negative and statistically significant impact, suggesting that tighter monetary conditions and higher borrowing costs discouraged investment. Furthermore, total savings was found to have a positive and statistically significant influence on aggregate investment expenditure, while real GDP, although positively related, was statistically insignificant, indicating a weak transmission from output growth to investment. The population growth rate, on the other hand, showed a negative and statistically significant effect on investment expenditure, reflecting the constraining pressure of demographic expansion on capital formation. Based on these findings, the study recommended that the Central Bank of Nigeria should have adopted a more accommodative interest rate framework through a gradual reduction in the monetary policy rate in order to reduce borrowing costs and stimulate private sector investment. It also recommended the implementation of a well-managed expansionary monetary policy, including reductions in bank reserve requirements and the maintenance of adequate liquidity in the financial system, to enhance credit availability and promote investment growth in the economy.