In this investigation, isolated pea root tips grown in sterile culture have been used to study the effects of the purine analogue, 8-azaguanine, upon growth during an 8-day period. The analogue is incorporated into the RNA of the root, but no significant incorporation into DNA is observed. It has been shown that cell division is rapidly inhibited while cell expansion is slightly increased. There is no selective inhibition of the synthesis of any single major cytoplasmic component, the synthesis of RNA, DNA and protein being inhibited to the same extent. The quantities of these constituents when expressed on a per cell basis are found to be greater in the presence of the analogue than in its absence. The reduction in the activity of the four enzymes studied, confirms that as a result of the incorporation of the analogue into RNA, some of the proteins synthesized lack their normal activity. It is suggested that the cessation of cell division is unlikely to be a consequence of the slight reduction in the synthetic capacity or metabolic activity of the roots and it is suggested that the incorporation of 8-azaguanine into a particular RNA component, possibly in the nucleolus, results in the blocking or alteration of a specific synthetic requirement for cell division.