  $^{75}$As NMR measurements were performed on a polycrystalline sample of
spin-1/2 alternating-spin-chain Heisenberg antiferromagnet AgVOAsO$_4$.
Temperature-dependent NMR shift $K(T)$, which is a direct measure of the
intrinsic spin susceptibility, agrees very well with the spin-1/2
alternating-chain model, justifying the assignment of the spin lattice. From
the analysis of $K(T)$, magnetic exchange parameters were estimated as follows:
the leading exchange $J/k_{\rm B} \simeq 38.4$ K, alternation ratio $\alpha =
J'/J \simeq 0.68$, and spin gap $\Delta/k_{\rm B} \simeq 15$ K. The transferred
hyperfine coupling between the $^{75}$As nucleus and V$^{4+}$ spins obtained by
comparing the NMR shift with bulk susceptibility amounts to $A_{\rm hf} \simeq
3.3$ T/$\mu_{\rm B}$. Our temperature-dependent spin-lattice relaxation rate
$1/T_1(T)$ also shows an activated behaviour at low temperatures, thus
confirming the presence of a spin gap in AgVOAsO$_4$.
