The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission by the National Aeronautics and Space Administration revealed that ~27% of the surface of Mercury is occupied by smooth plains potentially formed by magma eruptions. Most of these smooth plain units are located on the floors of the impact basins; these plains occupy a surface area that is approximately seven times larger in the northern hemisphere than in the southern hemisphere. This suggests a difference in thermal conditions in the mantle and/or the ease of magma eruption between hemispheres. This study re-estimated magma eruption ages and volumes within the Rembrandt and Caloris basins; these are representative impact basins in the southern and northern hemispheres, respectively. The estimations of eruption ages and volumes were carried out by measuring crater size-frequency distributions and the diameters of partially buried craters. The formation and eruption ages of the Rembrandt and the Caloris basins were estimated by adopting the porous-target chronology model; the formation ages were estimated as 3.93 ± 0.06 and 3.94 ± 0.04 Gy, while their eruption ages were from 3.87 ± 0.04 to 3.76 ± 0.01 Gy and 3.88 ± 0.03 to 3.74 ± 0.01 Gy, respectively. The observed crater size-frequency distributions in both basins might show multiple resurfacing from episodic eruptions, as suggested by previous numerical simulations of Mercurian thermal evolution. The estimated magma eruption fluxes in the Caloris basin (9.3–15.1 km/Gy) were at most, three times larger than that in the Rembrandt basin (5.2–12.3 km/Gy). Given its larger diameter, the crust beneath the Caloris basin is likely to be ~1.8 times thinner than the crust beneath the Rembrandt basin. This will make it easier for magma to ascend to the surface beneath Caloris than beneath Rembrandt; as such, quantities of magma production in the mantle beneath the two basins should not differ by a factor of <3. These results suggest that there are no large spatial variations in the abundance of heat-producing elements in the mantle and lower crust of Mercury unlike the concentration of heat-production elements on the nearside of the Moon.