Monte Carlo simulations of polymer chains in off-lattice systems under theta and athermal conditions have been performed in order to support results obtained for cubic lattice chains in assemblies of varying concentration. Global as well as local quantities are investigated in detail in order to point out tile differences as well, as the similarities of the results and the reliability of the two models, especially with respect to local packing effects, Furthermore, the results are discussed with reference to theoretical predictions of the scaling theory. Probably the most important finding is the fact that an isolated tl chain and a chain in bulk exhibit totally different microscopic structures (which is in full concordance with our previous lattice simulations), although the global dimensions of bulk polymers tend to converge to the values elf an isolated theta chain. (C) 1997 American Institute of Physics. [S0021-9606(97)51147-0].