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The retarded differential equation (I) x(t) = λx(t) + λF(x(t-1)), was studied. First, some applications on three biological models were mad. It was shown that for λ going to infinity, under mild conditions in F : R → R, i. e., F has stable orbit 2-periodic, the periodic solutions of (I), converge to "square wave" type function. Sequentially, studing the stability of the solutions, for F : Rⁿ → Rⁿ, it was shown that it only depends on the eigenvalues DF(0). Finally, applying additional hypothesis in F, it was shown the existence of periodical for λ > λ₀, being DF(0) a rotation folowed by a stretching, i.e. , the Jacobian matriz of DF(0) assumes one of the following forms: [ a b] / [-b a] or [a -b] / [b a], a > 0, b > 0, b > a, a² + b² > 1.