134. How many four-digit natural numbers without repeating digits can be written using only the digits $1,2,3,4,5$ and 6?
$\triangle$ A four-digit number can be represented as $\overline{x y z t}$, where $x, y, z, t$ are digits. Here, the digit $x$ can take 6 values, after which the digit $y-5$ (repetition of digits is not allowed), then the digit $z-4$, and the digit $t-3$. Therefore, the number of such four-digit numbers is $6 \cdot 5 \cdot 4 \cdot 3=360$.

Answer: 360.