Cytokinin-like activity and soluble sugar and starch levels were monitored during wheat grain (Triticum aestivum L.) and potato tuber (Solanum spp) development. Cytokinin-like activity was resolved on Sephadex LH-20 eluted with 35 or 20% ethanol and estimated in kinetin in equivalents from the soybean callus bioassay. The cytokinin-like activity increased in tubers larger than 5 - 7.5 mm diameter and reached a maximum in tubers 15 - 20 mm diameter. The amount per tuber was greatest in the largest size category analysed (40-50 mm diameter). The amount of sugar and starch per tuber also increased after tuber formation. There was a positive correlation between the highest concentration of cytokinin-like activity and the reported period of intense cell division, The cytokinin-like activity per pistil and per unit weight increased between ear emergence and pollination. On a per grain basis the activity increased to a high level 14 days after ear emergence (four days post-anthesis) but subsequently decreased to an undetectable level by 21 days. On a per unit weight basis the concentration was high but fluctuated between 10 and 14 days after ear emergence before decreasing. The most polar components of the activity were the O-glucosides of zeatin and zeatin riboside. These showed a rapid increase between ear emergence and anthesis but subsequently decreased, whereas zeatin increased most rapidly following pollination and reached a maximum four days post-anthesis. Zeatin riboside remained at a relatively low level at all stages of development. The highest level of cytokinin-like activity correlated with the reported onset of normal cell divisions in the endosperm. The amount of activity remained low as sugar and starch levels in the grain increased. Both zeatin and zeatin riboside were positively identified by GCMS (MIM) in extract of wheat grains.