Conditional statement

Dear useRs,

I wrote a function that simulates a stochastic model in discrete time.
The problem is that the stochastic parameters should not be negative  
and sometimes they happen to be.
How can I conditionate it to when it draws a negative number, it  
transforms into zero in that time step?

Here is the function:

stochastic_prost <- function(Fmean, Fsd, Smean, Ssd, f, s, n, time,  
out=FALSE, plot=TRUE) {
nt <- rep(0, time)
nt[1] <- n
for(n in 2:time) {
nt[n] <- 0.5*rnorm(1, Fmean, Fsd)*rnorm(1, Smean, Ssd)*exp(1)^(-(f 
+s)*nt[n-1])*nt[n-1]}
if(out==TRUE) {print(data.frame(nt))}
if(plot==TRUE) {plot(1:time, nt, type='l', main='Simulation',  
ylab='Population', xlab='Generations')}
}

The 2 rnorm()'s should not be negative; when negative they should  
turn into zero.

Thanks in advance,
Rafael