Processing of rRNA was examined in resting and phytohemagglutinin-stimulated guinea pig lymphocytes. Synthesis of 1.7 (28S) and 0.7 (18S) X 10(6) dalton rRNA was more than 4-fold greater in phytohemagglutinin-stimulated than in resting cells. A 5- to 10-fold increase in flux of molecules through a 2.3 X 10(6) dalton RNA occurred without a concurrent change in the flux through a 2.6 X 10(6) dalton fraction in phytohemagglutinin-stimulated cells. In both resting and phytohemagglutinin-stimulated lymphocytes, the 2.3 X 10(6) dalton intermediate equilibrated with [3H]methyl label and pulse-chased prior to the 2.6 X 10(6) dalton RNA. The data indicate at least two processing paths in guinea pig lymphocytes; one proceeds to rRNA via a 2.3 X 10(6) dalton intermediate, and another proceeds via a 2.6 X 10(6) dalton RNA. The increase in rRNA synthesis in phytohemagglutinin-stimulated cells occurs primarily through that path containing the 2.3 X 10(6) dalton intermediate.