The goa-1 gene encoding the alpha subunit of the heterotrimeric guanosine triphosphate-binding protein (G protein) G(o) from Caenorhabditis elegans is expressed in most neurons, and in the muscles involved in egg laying and male mating, Reduction-of-function mutations in goa-1 caused a variety of behavioral defects including hyperactive movement, premature egg laying, and male impotence, Expression of the activated G(o) alpha subunit (G alpha(o)) in transgenic nematodes resulted in lethargic movement, delayed egg laying, and reduced mating efficiency, Induced expression of activated G alpha(o) in adults was sufficient to cause these phenotypes, indicating that G alpha(o) mediates behavior through its role in neuronal function and the functioning of specialized muscles.