Plasma prolactin levels were determined in 16 healthy adult females before and after marihuana cigarette (1.83% delta 9-tetrahydrocannabinol) and placebo cigarette smoking. All subjects served as their own controls in double blind studies which were carried out on a residential research ward facility. Eight women were studied during the follicular phase and eight women were studied during the luteal phase of the menstrual cycle. Marihuana smoking did not produce any significant changes in plasma prolactin levels during the follicular phase of the menstrual cycle. However, plasma prolactin levels were significantly lower at 60 to 120 min (P less than .01) and 150 to 180 min (P less than .05) after marihuana smoking during the luteal phase of the menstrual cycle. Marihuana-induced suppression of plasma prolactin levels occurs in women but has not been observed in studies with human males.