  The complex transmission of Y_{1-x}Pr_xBa_2Cu_3O_7 single crystal thin films
has been measured in the range 0.2-1.0 THz using time domain spectroscopy. The
complex conductivity is calculated without using a Kramers-Kronig analysis. All
of the superconducting samples show a peak in $\sigma_1(T)$ below $T_c$. The
underdoped samples show a deviation from $1/(\alpha+\beta T)$ behavior above
$T_c$ that may be linked with the onset of a spin gap.
