A contemporary path model for the resolution of cultural and biological inheritance is extended to incorporate temporal variation in family resemblance. Specifically, the genetic and environmental effects, like all other parameters of the model, are allowed to vary over an individual's age according to some specific mathematical functions. In the computer program BETREND a library of such functions is incorporated. Data on systolic blood pressure in 542 Japanese-American nuclear families were analyzed. This new methodology detected significant temporal variation in cultural inheritance, a result that was not found previously by using static models which could only distinguish between adult and childhood heritabilities. Cultural heritability, estimated to be 10% at birth, increases to a maximum of 28% at age 36, at which time it declines until it eventually reaches 10% at age 49. Although not statistically significant, there was some evidence for temporal trends in genetic heritability as well. Ignoring trends, the genetic heritability was estimated as .30. On the other hand, trends in sibling environment were clearly nonsignificant. This model provides an objective method of testing the significance of temporal trends in familial resemblance by using multifactorial models, of resolving varying gene expression and transient environmental effects as possible sources of generating the observed temporal variation, and of estimating continuous changes in heritability with age.