  We obtain new explicit exponential stability conditions for linear scalar
equations with positive and negative delayed terms $$ \dot{x}(t)+ \sum_{k=1}^m
a_k(t)x(h_k(t))- \sum_{k=1}^l b_k(t)x(g_k(t))=0 $$ and its modifications, and
apply them to investigate local stability of Mackey--Glass type models
$$\dot{x}(t)=r(t)\left[\beta\frac{x(g(t))}{1+x^n(g(t))}-\gamma x(h(t))\right]$$
and $$\dot{x}(t)=r(t)\left[\beta\frac{x(g(t))}{1+x^n(h(t))}-\gamma
x(t)\right].$$
