The effect of increasing task difficulty on the threshold of rewarding, electrical brain stimulation was evaluated. Rats were trained to press a lever to obtain a brief burst of pulses to the lateral hypothalamus. The threshold was psychophysically scaled using a descending method of limits in which the pulse frequency was varied to yield a maximum to minimum range of self-stimulation rates. As expected, weighting the lever with 0, 15, 30, or 45 g produced progressive decreases in maximal rates, but it also caused a weight-related shift to the right of the rate-frequency ogives in each of the 7 rats. Although the degree of shift varied from rat to rat, it did not matter whether criterion performance was defined as half-maximum rate or as a constant rate of 5 responses. These results suggest that the effort required to make the operant response contributes to the position of rate-frequency curves and, further, that shifts in rate-frequency functions must be interpreted with caution when such shifts are obtained by CNS lesions or drug injections.