An incubator is provided for use with a chemical analyzer of the type in which sample fluids are metered onto analysis slides which are subjected to analyses. The incubator includes a temperature-controlled chamber having a rotor mounted therein which supports slides in a plurality of slide-carrying positions. The slide-carrying positions of the rotor are individually indexable to transfer locations wherein slides may be placed on the rotor or removed therefrom. Cover means supported by the rotor include a plurality of covers associated with the slide-carrying positions. The covers are positionable to an operative position to partially enclose a sample fluid to control evaporation. Control means are provided to selectively move a cover from the operative position to permit an analysis slide to be positioned in the transfer location of the incubator or to be removed therefrom, without disturbing the sample fluid on the slide.