Data from a long term field experiment were used to develop a predictive equation relating maize grain yield to nitrogen (N) and phosphate (P) applications, and total precipitation (RI) in different growth stages.of the maize plant. These stages were a pre-planting period, followed by a sequence of non-overlapping physiological growth phases from sowing to maturity. The equation was a mixed quadratic (N) and square-root (P) polynomial containing linear RI, Ri*N and RIP terms. The function was used to determine the response of maize grain yields to N and P, the fertilizer requirements for maximum profits and least cost N and P combinations. The effects of Low, Median, and High levels of rainfall were also investigated. It was found that yield response and profitability were adversely affected by increased levels of precipitation. The results indicate the importance of considering rainfall in fertilizer recommendations