I got my boy done not that long ago, i wouldnt have agreed to an overnight stay though unless there was a complication, considering the operation is fairly routine these days i would feel more comfortable getting it done by someone who would agree to relase them in the afternoon so i could care for him in the evening, i can see why a vet might want to keep them overnight as they are very groggy when you pick them up but within a few hours my boy was fine and i wouldnt have thought there was much point in him spending anymore time at the vets.
Cheers Andy
