  Recent Hubble Space Telescope (HST) observations by Rich et al. (1997) have
shown that the metal-rich globular clusters (GCs) NGC 6388 and NGC 6441 exhibit
a pronounced 2nd parameter effect. Ordinarily metal-rich GCs have only a red
horizontal-branch (HB) clump. However, NGC 6388 and NGC 6441 also possess an
unexpected population of blue HB stars, indicating that some 2nd parameter is
operating in these clusters. Quite remarkably, the HBs in both clusters slope
upward with decreasing B - V from the red clump to the top of the blue tail.
  We review the results of ongoing stellar evolution calculations which
indicate 1) that NGC 6388 and NGC 6441 might provide a crucial diagnostic for
understanding the origin of the 2nd parameter effect, 2) that differences in
age or mass loss along the red-giant branch (RGB) - the two most prominent 2nd
parameter candidates - cannot explain the HB morphology of these GCs, and 3)
that noncanonical effects involving an enhanced helium abundance or rotation
can produce upward sloping HBs. Finally we suggest a new metal-depletion
scenario which might help to resolve a baffling conundrum concerning the
surface gravities of the blue HB stars in these clusters.
