Four Gd3+ complexes [Gd(BP2A)(+), Gd(PC2A)(+), Gd(PCTA)(0), and Gd(BPO4A)(-)] with polyazamacrocyclic ligands that contain a pyridine moiety were prepared and examined for possible use as MRI contrast enhancement agents. We estimated the number of inner sphere water molecules (q(Gd)) for the Gd3+ complexes from the values of q found for the Tb3+ and/or Eu3+ complexes by luminescence lifetime measurements. We have estimated that q(Gd) = 3.5, 3.3, 2.4, and 0.2 for Gd(BP2A)(+), Gd(PC2A)(+), Gd(PCTP)(0), and Gd(BPO4A)(-), respectively. The inner sphere relaxivities (r(1,inner)) of these tetraaza macrocyclic complexes were higher than that of Gd(DOTA)(-) [i.e. 6.79 for Gd(BP2A)(+), 6.21 for Gd(PC2A)(+), and 4.60 for Gd(PCTA)(0) mM(-1)s(-1) at 40 MHz and 25 degrees C], but the normalized relaxivities per q(Gd) (1.94, 1.88, and 1.92 mM(-1) s(-1), respectively) were comparable to that of Gd(DOTA)(-). A quantitative treatment of the NMRD profiles based on Solomon-Bloembergen-Morgan theory, using the NMRD profile of Gd(BPO4A)(-) to correct for an outer sphere contribution, showed that the complexes exhibit characteristics similar to that of GP(DOTA)(-) but with shorter electronic relaxation times. Tissue biodistribution results using radioactive Sm-153 and Gd-159 complexes in rats indicate that the cationic [Sm-153(BP2A)(+) and Sm-153(PC2A)(+)] complexes accumulate preferably in the bone tissue while the neutral [Sm-153(PCTA)(0)] and anionic [Sm-153(BPO4A)-] complexes appear to have renal clearances similar to those of other low molecular weight contrast agents [i.e. Gd(DTPA)(2-) and Gd(DOTA)(-)].