Abstract. In a series of factorial experiments with cultivated Vitis californica Benth. (California wild grape) growth outdoors in full sun, we examined the effects of sunlight, temperature and water status on net CO2 uptake and PSH chlorophyll fluorescence at 77K. Exposure to either high light or high temperature caused reductions in PSH activity followed by partial or complete overnight recovery. Upon simulataneous exposure to high light and high temperature, PSH inhibition was severe and persistent. The maximum chlorophyll fluorescence (FM) and the ratio of variable to maximum fluorescence (Fv/FM) differed in their responses to combinations of light and temperature. At both low and high light. FM declined with increasing temperature over a wide temperature range, while Fv/FM exhibited a similar sensitivity to temperature only at high light. Net CO2 uptake declined by mid-afternoon and recovered by the next morning in most leaves, regardless of incident light or temperature. However, high-light leaves exhibited severe and lasting declines if temperatures exceeded 45°C. Water-stressed leaves exposed to high light exhibited greater reductions of net CO2 uptake than water-stressed leaves exposed to low light. However, the degree of light-dependent decline in PSH fluorescence (FM and Fv/FM) did not vary with water status, indicating that reduced PSH activity was not a primary cause of reduced carbon gain during water stress.