149 The unit digit of a positive integer $m$ is denoted by $f(m)$. $a_{n}=f\left(2^{n+1}-1\right)(n=1,2, \cdots)$, then $a_{1994}$ $=$ ـ. $\qquad$
$149 \quad 7 . \quad 1994=4 \times 498+2$,
$$
\begin{aligned}
a_{1994} & =f\left(2^{1 \times 498-3}-1\right)=f\left(2^{4 \times 498} \times 8-1\right) \\
& =f\left(16^{198} \times 8-1\right)=f(6 \times 8-1)=7 .
\end{aligned}
$$