The effect of natural air temperature on the vegetative growth and alkaloidal yield of H. muticus L. was studied. The highest number of branches, diameter, fresh and dry weights of stem and roots occurred in September sowing at full‐flowering and fruiting stages. Relatively higher temperature in Sept. sowing favoured leaf growth (number, area and dry weight). Hyoscyamine yield in different organs of the plants sown in Sept., excessively exceeded all other sowings during flower budding and full‐flowering stages. Hyoscine yield was quantitatively very small as compared with that of hyoscyamine and even could not be detected in the leaves of winter sowings during full‐flowering stage.

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