This paper reports the results of a large scale randomized experiment that was de- signed to assess whether a short in-class intervention by an external female role model can influence students’ attitudes towards science and contribute to a significant change in their choice of field of study. The intervention consists in a one hour, one off visit of a high school classroom by a volunteer female scientist. It is targeted to change students’ perceptions and attitudes towards scientific careers and the role of women in science, with the aim of ultimately reducing the gender gap in scientific studies. Using a random as- signment of the interventions to 10th and 12th grade classrooms during normal teaching hours, we find that exposure to female role models significantly reduces the prevalence of stereotypes associated with jobs in science, for both female and male students. While we find no significant effect of the classroom interventions on 10th grade students’ choice of high school track the following year, our results show a positive and significant impact of the intervention on the probability of applying and of being admitted to a selective science major in college among 12th grade students. This effect is essentially driven by high-achieving students and is larger for girls in relative terms. After the intervention, their probability to be enrolled in selective science programs after graduating from high school increases by 30 percent with respect to the baseline mean.