Phosphorus ( 31 P) T 1 and T 2 relaxation times in the resting human calf muscle were assessed by interleaved, surface coil localized inversion recovery and frequency-selective spin-echo at 3 and 7 T. The obtained T 1 (mean ؎ SD) decreased significantly (P < 0.05) from 3 to 7 T for phosphomonoesters (PME) (8.1 ؎ 1.7 s to 3.1 ؎ 0.9 s), phosphodiesters (PDE) (8.6 ؎ 1.2 s to 6.0 ؎ 1.1 s), phosphocreatine (PCr) (6.7 ؎ 0.4 s to 4.0 ؎ 0.2 s), ␥-NTP (nucleotide triphosphate) (5.5 ؎ 0.4 s to 3.3 ؎ 0.2 s), ␣-NTP (3.4 ؎ 0.3 s to 1.8 ؎ 0.1 s), and -NTP (3.9 ؎ 0.4 s to 1.8 ؎ 0.1 s), but not for inorganic phosphate (Pi) (6.9 ؎ 0.6 s to 6.3 ؎ 1.0 s). The decrease in T 2 was significant for Pi (153 ؎ 9 ms to 109 ؎ 17 ms), PDE (414 ؎ 128 ms to 314 ؎ 35 ms), PCr (354 ؎ 16 ms to 217 ؎ 14 ms), and ␥-NTP (61.9 ؎ 8.6 ms to 29.0 ؎ 3.3 ms). This decrease in T 1 with increasing field strength of up to 62% can be explained by the increasing influence of chemical shift anisotropy on relaxation mechanisms and may allow shorter measurements at higher field strengths or up to 62% additional signal-to-noise ratio (SNR) per unit time. The fully relaxed SNR increased by ؉96%, while the linewidth increased from 6.5 ؎ 1.2 Hz to 11.2 ؎ 1.9 Hz or ؉72%. Key words: phosphor; spectroscopy; human muscle; relaxation times; 7 Tesla; high field; chemical shift anistrophy Phosphorus ( 31 P) MR spectroscopy (MRS) is a powerful tool for the noninvasive investigation of human muscle metabolism under various physiological and pathological conditions (1). High-field MR systems, i.e., 7 T, offer advantages to 31 P-MRS in terms of sensitivity and spectral resolution, which has recently been shown in the brain (2).To optimize the measurement parameters of clinical spectroscopy protocols, such as echo time (TE) and repetition time (TR), an accurate knowledge of T 1 and T 2 relaxation times is essential. With TR chosen on the order of T 1 , severe saturation effects must be taken into account, in addition to T 2 decay. For absolute quantification of metabolites, it is therefore essential to accurately determine relaxation times to allow subsequent corrections for T 1 and T 2 decay (3,4).Relaxation times vary not only between different metabolites, but also with B 0 . The T 1 and T 2 of 31 P metabolites in the human leg were previously determined only at lower field strengths, i.e., at 1.5 T (5-13), 2.0 T (14), 2.35 T (15), and 3 T (16), while in vivo data at higher fields have been obtained only from animal studies (17,18).Relaxation in 1 H-MRS is dominated by magnetic dipoledipole interactions according to the Bloembergen-Purcell-Pound (BPP) theory (19). Therefore, 1 H-MRS T 1 relaxation times are increasing with B 0 (20). However, for 31 P-MRS, both dipolar relaxation and chemical shift anisotropy (CSA) are the two major, competing relaxation mechanisms (17,(21)(22)(23). In contrast to dipolar interaction, the contributions of CSA to 1/T 1 and 1/T 2 relaxation rates are proportional to the gyromagnetic ratio (␥), B 0 2 , the asymmetry of the magnetic shielding (...