It is shown in this letter that the field-effect electrical response of transistors with their channel made of networks of single-walled carbon nanotubes (SWNTs) embedded in a poly(3-hexylthiophene) (P3HT) matrix can be significantly altered by light illumination. The experimental results indicate a photo-activated electron transfer from P3HT selectively to the semiconducting SWNTs. This finding points to a potential optoelectronic application of such a field-effect device as a photo-triggered electronic switch.