for many spiral galaxies (e.g., Persic, Salucci & Stel 1996), these profiles constrain primarily the properties of the dark halo. Considerable uncertainty remains on the $M/L$ of the stellar population, or alternatively, on whether spiral galaxy disks are “maximal” or not (e.g., van Albada [[et al. ]{}]{}1985; Courteau & Rix 1999).

Uncertainties in the IMF for low-mass stars cause considerable uncertainty in the predicted $M/L$ at fixed age. So while accurate dynamical $M/L$ measurements are available for many early-type galaxies, these do not uniquely constrain the age of their stellar population. A more powerful constraint is provided by the [*relative*]{} rate at which the $M/L$ varies with time, or alternatively, redshift: $[d(M/L) / (M/L)] / dz \propto d \log (M/L) / dz$. Because this is a relative measure, it depends less strongly on uncertainties in the IMF for low-mass stars. Of course, a dependence does remain on the exact shape of the IMF for the higher-mass stars that produce most of the light. Nonetheless, $d \log (M/L) / dz$ can be used to meaningfully constrain the formation redshift of early-type galaxies (van Dokkum [[et al. ]{}]{}1998).

Studies of $d \log (M/L) / dz$ have so far relied primarily on measurements 