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SUMMARY The effects of photoperiod (8 or 16 hr, SD or LD) and temperature (low or high, 8° or 20°C) on the growth and cytokinin content of Norwegian and Portuguese populations of Dactylis glomerata (Cocksfoot) were investigated. In general, conditions which promote active leaf growth (LD or high temperature) of plants of the Norwegian population tend to result in lower levels of cytokinin bases and/or nucleosides and in higher levels of ‘nucleotide’ cytokinins than conditions which lead to reduced growth rate (SD or low temperature). In some ‘transfer’ experiments the changes in hormone levels preceded detectable changes in growth rate. Cytokinins which behaved similarly to zeatin and zeatin riboside on Sephadex LH 20 columns were detected. The lack of response to application of exogenous zeatin or 6‐benzylaminopurine, together with the data on endogenous cytokinins, suggests that the level of cytokinin bases and nucleosides was not limiting the growth rate of the Norwegian population in SD at 8°C. However, if ‘nucleotide’ cytokinins are significant for growth rate under unfavourable conditions then this class of hormone may be involved in the responses to day length and temperature.