  We consider the transport equation $\ppp_tu(x,t) + (H(x)\cdot \nabla u(x,t))
+ p(x)u(x,t) = 0$ in $\OOO \times (0,T)$ where $\OOO \subset \R^n$ is a bounded
domain, and discuss two inverse problems which consist of determining a
vector-valued function $H(x)$ or a real-valued function $p(x)$ by initial
values and data on a subboundary of $\OOO$. Our results are conditional
stability of H\"older type in a subdomain $D$ provided that the outward normal
component of $H(x)$ is positive on $\ppp D \cap \ppp\OOO$. The proofs are based
on a Carleman estimate where the weight function depends on $H$.
