Introduction: Greater reduction in estimated glomerular filtration rate (eGFR) after specific treatment for primary aldosteronism (PA) reflects improvement in glomerular hyperfiltration associated with PA and leads to better patient outcomes. However, little is known regarding the mechanisms underlying eGFR reduction after treatment for PA. Methods: We analyzed data from the nationwide PA registry in Japan. Patients were assigned to adrenalectomy (n = 438) and mineralocorticoid receptor (MR) antagonist (n = 746) groups. We assessed associations between changes in blood pressure (BP), plasma renin activity (PRA) and plasma aldosterone concentrations (PAC), and eGFR before and 6 months after treatment for both groups. Results: In a multivariable linear regression, the adjusted β values (95% confidence interval [CI]) for change in eGFR after treatment were -2.76 (-4.29, -1.22) ml/min per 1.73 m2 for PRA (per 3.2 ng/ml per hour), and 1.97 (1.08, 2.85) ml/min per 1.73 m2 for PAC (per 236.1 pg/ml) in the adrenalectomy group; and -0.45 (-0.89, -0.01) ml/min per 1.73 m2 for PRA and -0.72 (-1.62, 0.18) ml/min per 1.73 m2 for PAC in the MR antagonist group. Change in mean arterial pressure after treatment was not significantly associated with change in eGFR in either group. Changes in PRA and PAC but not BP before and 6 months after treatment for PA were associated with greater reductions in eGFR. Conclusion: Post-treatment improvements in glomerular hyperfiltration may be attributable to decreased MR activity in the kidneys, but not to reductions in systemic BP.