I have a large dataset but for this analysis I am focusing on three columns that includes (a) the concentration of hormone X (ug/g), (b) body mass (kg) and gender (male/female).

I am just wondering how best to approach the analysis and I thought a mixed effect model was most appropriate. That is, the concentration of hormone X increases with body mass however it is also dependent on gender.

I tried using this code:

model2=lme(conc.hormone~gender*Body.Mass..kg.,
data=epidemiologythesisdata,
 na.omit = TRUE)

...however I received an error. Is there a way that I can graph the relationship between hormone concentration and body mass with two different lines for gender?

Thanks