[en] The local magnetic fields and spin dynamics of the itinerant helimagnet MnSi(T_c ≅ 29.5 K) have been studied experimentally using positive muon spin rotation/relaxation (μ⁺SR) methods. In the ordered phase (T < T_c), zero-field μSR was used to measure the hyperfine fields at the muon sites as well as the muon spin-lattice relaxation time T₁^μ. Two magnetically inequivalent interstitial μ⁺ sites were found with hyperfine coupling constants A_hf⁽¹⁾ = -3.94 kOe/μ_B and A_hf⁽²⁾ = -6.94 kOe/μ_B, respectively. In the paramagnetic phase (T > T_c), the muon-nuclear spin double relaxation technique was used to simultaneously but independently determine the spin-lattice relaxation time T₁^Mn of ⁵⁵Mn spins and that of positive muons (T₁^μ) over a wide temperature range (T_c < T ≤ 150 K). The temperature dependence of T₁^Mn and T₁^μ in both phases shows systematic deviations from the predictions of self-consistent renormalization (SCR) theory. (author)