The changes in the levels of five endogenous gibberellins (GAs) in spinach in relation to photoperiodic treatment have been examined by combined gas chromatography-selected ion current monitoring. Long-day treatment caused a 5-fold decline in the level of GA(19) while the levels of GA(20) and GA(29) increased dramatically during the same period. In absolute terms, the level of GA(20) increased from 0.8 microgram per 100 grams dry weight in short days to 5.5 micrograms per 100 grams dry weight after 14 long days. The levels of GA(17) and GA(44) did not change significantly with long-day treatment. These results are consistent with the hypothesis that GA(19) is converted to GA(20) and that this conversion is under photoperiodic control. Since stem growth in spinach is correlated with an increase in the level of GA(20), one major aspect of photoperiodic control of stem growth might be the availability of GA(20) through regulation of the conversion of GA(19) to GA(20).