The effect of cyclic nucleotides on collagen production by human intestinal smooth muscle cells was examined in vitro. Cholera toxin and isobutylmethylxanthine, agents that elevate cyclic adenosine monophosphate, caused selective inhibition of collagen production when cells were exposed to these agents for 24-72 h. Exposure for 6 h inhibited noncollagen protein synthesis without effects on collagen production. Forskolin similarly inhibited collagen production, decreasing relative collagen synthesis 40% at 10 microM and 60% at 100 microM. After 48 h of exposure to cholera toxin and isobutylmethylxanthine, levels of cyclic adenosine monophosphate had increased in a concentration-dependent manner. The effect of cyclic nucleotide analogues was also examined. Dibutyryl cyclic adenosine monophosphate inhibited collagen production, whereas dibutyryl cyclic guanosine monophosphate increased collagen production by 65%. This effect was maximal at a concentration of 10 microM. These observations suggest that cyclic adenosine monophosphate has a significant effect on collagen production by human intestinal smooth muscle cells in vitro and may play a role in the modulation of collagen production by these cells in vivo.