NATIONAL ZOO in Washington D.C. and maybe others -

Most people presume or may have read somewhere that the opening time for the zoo is 10 in the morning. This is unfortunate. By that hour most of the animals have already been fed and are feeling rather lethargic. More often than not they will soon be trying to find a shady spot to spend the majority of the rest of the day. I haven't been to this zoo for a couple of years, but I think it is safe to say that is well worth knowing that visitors are not turned away if they arrive early. Try getting there at about 6:30 or 7. The air will be filled with every manner of animal sounds, most particularly that of the yet-to-be-fed big cats. Their roars are awesome! Nearly all of the animals will be much more active and far less likely to be tucked away where you can't see them. Actual opening times are:

May 1 to September 15:
Grounds    6 a.m. - 8 p.m.
Buildings 10 a.m. - 6 p.m.

September 16 to April 30 :
Grounds    6 a.m. - 6 p.m.
Buildings 10 a.m. - 4:30 p.m.