The efficiency of energy transfer between a fluorescent donor, L-tyrosine, and a fluorescent acceptor, L-tryptophan, has been determined in R'-L-Trp-L-Ala-L-Tyr-R", R-L-Trp-L-Ala-L-Ala-L-Tyr-R", and R'-L-Trp-Gly-L-Ala-L-Tyr-R" in ethanol solution. The protecting groups R' and R" were respectively tert-butyloxycarbonyl and methyl ester. A conformational theoretical analysis of molecules studied has been performed in parallel on the basis of semiempirical conformational potential energy function. In the theoretical models all the side chains have been represented by a methyl group. From the distribution of distances between chromophores obtained theoretically, transfer efficiencies have been computed assuming a random orientation of the chromophores (k-2 equals 2/3). The comparison of calculated efficiencies with the values determined experimentally for the same value of k-2 has been used as a check for the theoretical model. Both experimental and theoretical studies have been shown that the glycyl residue procudes a reduction of dimensions when it replaces in a tetrapeptide a residue with a beta-carbon atom such as the L-alanyl residue. However, only a qualitative agreement between experimental and theoretical values of the efficiencies has been obtained.