PURPOSE: Phospholipase C (PLC) isozymes ply significant roles in transmembrane signal transduction. PLC-i 1 acts as the intracellular effector in signal transduction for cellular proliferation and differentiation. Ras oncoprotein is also involved in cell growth. We determined the biological significance of PLC and ras oncoprotein in regeneration following radiation and the effect of different modes of administration of 5-FU. MATERIALS AND METHODS: To determine the effect of the administration mode of 5-FU on the regeneration of intestinal mucosa of rats following radiation, we compared the expression of PLC and ras oncoprotein in six groups. Group I had no treatment. GroupII received radiation( 8 Gy) only. Group III received radiation(8 Gy) and 5-FU(150mg/kg) continuous intravenous (iv) infusion for 12 hours. Group iv bolus injection. Group V received only 5-FU(150mg/kg) iv bolus injection. Through immunoblotting and immunohistochemistry, we examined the expression of PLC and ras oncoprotein in rat jejunum at 96 hours after radiation or 5-FU administration and at 120 hours after radiation and 5-FU administration. We also investigated the histological findings using hematoxylin and eosin stain. RESULTS: In the immunohistochemistry study, PLC-i 1 expression was the highest in group III followed by groups II and VI in that order and was weakly positive in groups V and VI. PLC-i 1 was hardly detected in the control group. The expression of ras oncoprotein wss the same as the PLC-i 1 expression for all groups. These results were confirmed by the histological findings regarding the mucosal regeneration. In the immunoblotting analysis, PLC-i 1 expression was the highest in group III followed by group IV and II in that order. This difference between the immunoblotting and immunohistochemistry study was due to the high expression of PLC-i 1 on the damaged surface epithelium rather than to its expression in the regeneration region as observed in the immunohistochemistry study for group IV. The expression of PLC- 1 was positive only in group V and VI, which received both radiation and 5-FU, and the expression of PLC-b 1was negligible for all groups. CONCLUSION: These results suggest that PLC-i 1 mediated signal transduction and ras oncoprotein may have a significant role in mucosal regeneration after radiation, and that continuous iv infusion of 5-FU may induce active regeneration in intestinal mucosa following radiation. In addition, the expression of PLC- 1 in combined group of radiation and 5-FU implies that PLC- 1 may be involved in signal transduction mediated by concerted action between radiation and 5-FU.