Single-unit, extracellular recordings were made from spontaneously active, thermosensitive neurons in the preoptic region of green sunfish acclimated to 25 degrees C. Activity of single cells was monitored during increases and decreases in local brain temperature over approximately a 10 degrees C range. Deep-body and skin temperatures were maintained at 25 degrees C. Of 276 neurons, 81% were insensitive, 17% were warm sensitive, and 2% cold sensitive. Warm responses were grouped into three basic types: exponential, linear, and nonlinear. All cold-sensitive neurons responded in a similar nonlinear manner. Mean levels of firing rate of thermosensitive neurons at 25 degrees C brain temperature ranged from 6 +/- 1.1 impulses/s to 22.7 +/- 10.8 impulses/s. Thermosensitivities were as high as 5.2 +/- 0.9 impulses . -1 . degrees C-1. Anatomic location of these neurons within the preoptic region appears random with some trend for the exponentially responding, warm-sensitive neurons to be located more medial than the other thermosensitive cell types. A small number of neurons were located in the ventrolateral telencephalon. In general, the thermosensitive responses observed resemble those found in other ectotherms and mammals with some exceptions.