Objectives. Grip strength and reasoning are associated in old age. This is one of the few longitudinal studies addressing whether aging of one causes decline in the other or whether they share causal influences. Methods. The Lothian Birth Cohort 1921 were assessed for grip strength and nonverbal reasoning at ages M = 79 (N = 550), M = 83 (N = 321), and M = 87 (N = 207). Associations among intercepts and slopes for grip strength and reasoning and covariates were examined by fitting a bivariate growth curve structural equation model. Results. Grip strength and reasoning declined with age. They were each significantly correlated on each occasion. Their intercepts were significantly correlated (.20) but not their slopes. Neither intercept was significantly associated with its own or the other's slope. Better reasoning was associated with higher childhood intelligence, more professional occupations, male sex, and being taller. There were no significant reasoning slope associations. Stronger grip strength was associated with male sex, being taller, and drinking less alcohol. Women showed less age-related decline in grip strength. Discussion. Physical and mental "grips" declined in the ninth decade of life. Their levels were significantly correlated; their slopes were not. There was no evidence for reciprocal dynamic influences nor for shared associations.