Pyridine-2,6-dicarboxylic acid (pdcH(2)) reacts with nitrate salts of La(III), Ce(III), and Nd(III) under hydrothermal conditions to form three-dimensional network structures, 1-4. All the compounds crystallize in the monoclinic space group P2(1)/c with the following lattice parameters: {La-2(pdc)(3).3H(2)O} 1, a = 10.966(3) Angstrom, b = 17.534(4) Angstrom, c = 13.578(2) Angstrom, = 100.23(3)degrees, V = 2569.3(g) Angstrom(3), Z = 4, R1 = 0.052, wR2 = 0.143, S = 1.06; {Ce-2(pdc)(3).3H(2)O} 2, a = 12.701(3) Angstrom, b = 9.979(1) Angstrom, c = 19.401(4) Angstrom, beta = 97.73(2)degrees, V = 2436.6(8) Angstrom(3), Z= 4, R1 = 0.039, wR2 = 0.117, S = 1.30; {Ce-2(pdc)(3).3H(2)O} 3, a = 10.961(5) Angstrom, b = 17.523(5) Angstrom, c = 13.505(2) Angstrom, beta = 100.89(3)degrees, V = 2547.2(8) Angstrom(3), Z = 4, R1 = 0.040, wR2 = 0.103, S = 1.08; {Nd-2(pdc)(3).3H(2)O} 4, a 10.944(3) Angstrom, b = 17.448(5) Angstrom, c = 13.397(2) Angstrom,beta = 101.19(3)degrees, V= 2569.3(g) Angstrom(3), Z= 4, R1 = 0.052, wR2 = 0.143, S = 1.06. Compounds 1, 3, and 4 form infinite single helical chains with large widths and pitches containing four metal ions per turn while 2 forms a different helix with smaller width and pitch containing two metal ions per turn. Each helix is further bonded on either side via carboxylate bridging producing three-dimensional metal-organic framework structures.