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In the nematode C. elegans, heterotrimeric G proteins have been shown to regulate the behavior of locomotion, feeding and egg-laying. Go belongs to Gi family and only exists in organisms with a nervous system. The signaling downstream of Go has been a mystery since its discovery, and it is what we are determined to find out. Loss of Goα causes animals to be hyperactive and lay eggs constitutively. Overexpressing Goα causes opposite phenotypes. Another α protein a subunit, Gq, causes phenotypes opposite to Go. To identify G protein effectors in C. elegans, we performed a forward genetic screen for suppressors of activated Goα under the control of the heat-shock promoter hsp16-2. Because of the nature of the screen design, we identified two categories of genes. One category acts on heat shock response and the other category acts on G protein pathways. We characterized and positional cloned genes from both categories. The second chapter of the thesis described sag-4, a cyclin L homologue that specifically affects heat shock promoters and decreases heat-shock induced protein expression. We propose that cyclin Lis likely to be involved in heat shock induced transcription. Other genes in this category, sag-3, sag-5 and sag-8, may also function in similar mechanisms. The third chapter of the thesis focused on G protein signaling. eat-16 was identified in the screen for suppressors of activated Goα. We positional cloned it and found it encodes a RGS7 homologue. RGS proteins have been studied as GTPase Activating Protein for the α subunits of heterotrimeric G proteins. Although eat-16 was identified in a suppressor screen for activated Go (goa-1), both genetic and biochemical evidence showed that eat-16 is a GAP for Gq (egl-30). We propose that Go and Gq antagonize each other, thereby regulating behaviors. Go might negatively regulate Gq signaling, possibly through eat-16 or other unrevealed genes. Chapter four describes our reconstituted system in mammalian cell culture. EAT-16 decreases Gq/G11 mediated PLC activity. GOA-1 and GPB-2 (C. elegans Gβ5 homologue) also decrease PLC activity induced by Gq/G11. These results are consistent with the hypothesis that Go negatively regulate Gq signaling, and the interaction between Gβ5 and RGS7 can be one of the steps between Go and Gq.