A solid state temperature regulator is provided for a solar heating system for use in conjunction with a swimming pool, or the like. The solar swimming pool heating system includes the usual components, namely, a pump, a filter, and a collector, and in which the pump serves to circulate the water from the pool through the filter and collector and back into the pool. The system also includes additional components, namely, temperature sensors for the collector and for the circulated pool water, appropriate valves, and a solid state control circuit. The solid state control circuit responds to predetermined temperature differences sensed by the sensors to cause the pool water to be circulated through the collector so long as the collector is at a higher temperature than the circulated pool water, and which causes the circulated pool water to by-pass the collector when the temperature of the collector drops below the temperature of the circulated pool water. The control circuit also has a high temperature cut-off control which activates the valves to cause the circulated pool water to by-pass the collector when the temperature of the circulated pool water exceeds a particular threshold. The control circuit also includes a mode switch which may be actuated to reverse the action of the system, causing the pool water to be circulated through the collector when the collector temperature is lower than the pool water temperature, for example, at night following a hot day, in which the collector radiates to the black sky, whereby the collector can be used to cool the water in the pool.