The crystal structures of p-methoxyphenyl-3-triazolo [4,3-a] isoquinoline (SR95926), p-methoxyphenyl-3-triazolophtalazine (CMW1842), and p-methoxyphenyl-3-N-dimethoxyethylamino-6-triazolophtalazine (L16317) have been solved by direct methods from single-crystal X-ray diffraction data, and refined by full-matrix least squares. SR95926: monoclinic, P21/n, a=20.950(3), b=6.769(1), c=9.465(2) Å, β=100.90(1)°. CMW1842: triclinic, P-1, a=8.784(1), b=9.160(4), c=8.555(1) Å, α=99.10(2), β=93.90(1), γ=106.77(1)°. L16317: monoclinic, P21/n, a=20.124(3), b=9.586(1), c=10.788(1) Å, β=91.91(1)°. Final R factors are 0.034, 0.037, and 0.053, respectively. Experimental geometries were used to perform STO-3G ab initio molecular-orbital calculations. A relationship between the electronic pattern within the molecules and the affinity of the benzodiazepine receptor sites is pointed out. © 1991 Plenum Publishing Corporation.